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High School Mathematics Extensions From Wikibooks, the open-content textbooks collection Note: current version of this book can be found at http://en.wikibooks.org/wiki/High_school_extensions" Contents HIGH SCHOOL MATHEMATICS EXTENSIONS ..............................................................................................................1 CONTENTS .................................................................................................................................................................2 AUTHORS ...................................................................................................................................................................4 PRIMES AND MODULAR ARITHMETIC .........................................................................................................................5 Primes ..................................................................................................................................................................5 Introduction..........................................................................................................................................................5 Factorisation.........................................................................................................................................................6 2, 5 and 3..............................................................................................................................................................8 Theorem - Divisibility by 3..................................................................................................................................8 Finding primes .....................................................................................................................................................9 Infinitely many primes .......................................................................................................................................10 Modular Arithmetic............................................................................................................................................12 To go further ......................................................................................................................................................33 Acknowledgement..............................................................................................................................................33 Reference ...........................................................................................................................................................34 Feedback ............................................................................................................................................................34 Problem Set ........................................................................................................................................................34 Square root of minus 1 .......................................................................................................................................37 Project -- The Square Root of -1 ........................................................................................................................37 SOLUTIONS TO EXERCISES .......................................................................................................................................39 HSE Primes|Primes and Modular Arithmetic.....................................................................................................39 Diophantine equation Exercises .........................................................................................................................45 Problem set solutions .........................................................................................................................................50 DEFINITIONS ............................................................................................................................................................54 LOGIC ......................................................................................................................................................................54 Introduction........................................................................................................................................................54 Boolean algebra..................................................................................................................................................54 Propositions........................................................................................................................................................69 Logic Puzzles .....................................................................................................................................................72 Problem Set ........................................................................................................................................................74 Feedback ............................................................................................................................................................75 MATHEMATICAL PROOFS .........................................................................................................................................76 Introduction........................................................................................................................................................76 Mathematical induction......................................................................................................................................76 Proof by contradiction........................................................................................................................................79 Reading higher mathematics ..............................................................................................................................81 Axioms and Inference ........................................................................................................................................83 Problem Set ........................................................................................................................................................86 Feedback ............................................................................................................................................................87 EXERCISES ...............................................................................................................................................................88 Mathematical proofs...........................................................................................................................................88 Problem set.........................................................................................................................................................92 Mathematical Proofs Problem Set......................................................................................................................92 INFINITY AND INFINITE PROCESSES ..........................................................................................................................95 Introduction........................................................................................................................................................95 Infinite Sets ........................................................................................................................................................95 Limits Infinity got rid of ..................................................................................................................................102 Infinite series....................................................................................................................................................105 2 Feedback ..........................................................................................................................................................106 Exercises ..........................................................................................................................................................107 Infinity and infinite processes ..........................................................................................................................107 Problem set.......................................................................................................................................................109 COUNTING AND GENERATING FUNCTIONS .............................................................................................................110 Some Counting Problems.................................................................................................................................110 Generating functions ........................................................................................................................................110 Further Counting ..............................................................................................................................................116 Problem Set ......................................................................................................................................................121 Feedback ..........................................................................................................................................................125 Exercises ..........................................................................................................................................................126 Counting and Generating functions..................................................................................................................126 DISCRETE PROBABILITY ........................................................................................................................................132 Introduction......................................................................................................................................................132 Event and Probability.......................................................................................................................................132 Random Variables............................................................................................................................................136 Areas as probability..........................................................................................................................................143 Order Statistics.................................................................................................................................................143 Addition of the Uniform distribution ...............................................................................................................143 Feedback ..........................................................................................................................................................143 FINANCIAL OPTIONS ..............................................................................................................................................144 Binary tree option pricing ................................................................................................................................144 Call-Put parity ..................................................................................................................................................150 Reference .........................................................................................................................................................150 Feedback ..........................................................................................................................................................150 MATRICES .............................................................................................................................................................151 Introduction......................................................................................................................................................151 Matrix Multiplication .......................................................................................................................................153 The Identity & multiplication laws ..................................................................................................................161 Determinant and Inverses.................................................................................................................................162 Other Sections ..................................................................................................................................................168 Exercises ..........................................................................................................................................................168 Matrices............................................................................................................................................................168 Problem set.......................................................................................................................................................177 Matrices Problem Set .......................................................................................................................................177 FURTHER MODULAR ARITHMETIC .........................................................................................................................180 Introduction......................................................................................................................................................180 Modular Arithmetic with a general m ..............................................................................................................182 Two-torsion Factorisation ................................................................................................................................191 MATHEMATICAL PROGRAMMING ...........................................................................................................................193 Introduction to programming ...........................................................................................................................193 Discrete Programming .....................................................................................................................................194 Feedback ..........................................................................................................................................................196 BASIC COUNTING ...................................................................................................................................................198 Counting...........................................................................................................................................................198 PARTIAL FRACTIONS ..............................................................................................................................................201 Method of Partial Fractions..............................................................................................................................201 More on partial fraction ...................................................................................................................................204 SUMMATION SIGN ..................................................................................................................................................210 Summation Notation ........................................................................................................................................210 Operations of sum notation ..............................................................................................................................211 Beyond .............................................................................................................................................................212 COMPLEX NUMBERS ..............................................................................................................................................213 Introduction......................................................................................................................................................213 3 Arithmetic with complex numbers...................................................................................................................216 The complex root .............................................................................................................................................219 The complex plane ...........................................................................................................................................221 de Moivre's Theorem .......................................................................................................................................224 Complex root of unity ......................................................................................................................................225 PROBLEM SET ........................................................................................................................................................226 DIFFERENTIATION..................................................................................................................................................233 Differentiate from first principle ......................................................................................................................233 LICENSE .................................................................................................................................................................247 GNU Free Documentation License ..................................................................................................................247 0. PREAMBLE ................................................................................................................................................247 1. APPLICABILITY AND DEFINITIONS.....................................................................................................247 2. VERBATIM COPYING ..............................................................................................................................248 3. COPYING IN QUANTITY .........................................................................................................................248 4. MODIFICATIONS ......................................................................................................................................248 5. COMBINING DOCUMENTS.....................................................................................................................249 6. COLLECTIONS OF DOCUMENTS...........................................................................................................249 7. AGGREGATION WITH INDEPENDENT WORKS .................................................................................250 8. TRANSLATION..........................................................................................................................................250 9. TERMINATION..........................................................................................................................................250 10. FUTURE REVISIONS OF THIS LICENSE .............................................................................................250 External links ...................................................................................................................................................250 Authors Zhuo Jia Dai, User:R3m0t, Martin Warmer, Tom Lam. 4 Primes and modular arithmetic Primes Introduction A prime number (or prime for short) is a natural number that can only be wholly divided by 1 and itself. For theoretical reasons, the number 1 is not considered a prime (we shall see why later on in this chapter). For example, 2 is a prime, 3 is prime, and 5 is prime, but 4 is not a prime because 4 divided by 2 equals 2 without a remainder. The first 20 primes are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71. Primes are an endless source of fascination for mathematicians. Some of the problems concerning primes are so difficult that even decades of work by some of the most brilliant mathematicians have failed to solve them. One such problem is Goldbach's conjecture, which states that all even numbers greater than 3 can be expressed as the sum of two primes. Geometric meaning of primes Let's start with an example. Given 12 pieces of square floor tiles, can we assemble them into a rectangular shape in more than one way? Of course we can, this is due to the fact that We do not distinguish between 2×6 and 6×2 because they are essentially equivalent arrangements. But what about the number 7? Can you arrange 7 square floor tiles into rectangular shapes in more than one way? The answer is no, because 7 is a prime number. Fundamental Theorem of Arithmetic A theorem is a non-obvious mathematical fact. A theorem must be proven; a proposition that is generally believed to be true, but without a proof, is called a conjecture. With those definitions out of the way the fundamental theorem of arithmetic simply states that: Any natural number (except for 1) can be expressed as the product of primes in one and only one way. For example 5 Rearranging the multiplication order is not considered a different representation of the number, so there is no other way of expressing 12 as the product of primes. A few more examples A number that can be factorised into more than 1 prime factor is a called a composite number (or composite for short). Composite is the opposite of prime. Think about it Bearing in mind the definition of the fundamental theorem of arithmetic, why isn't the number 1 considered a prime? Factorisation We know from the fundamental theorem of arithmetic that any integer can be expressed as the product of primes. The million dollar question is: given a number x, is there an easy way to find all prime factors of x? If x is a small number that is easy. For example 90 = 2 × 3 × 3 × 5. But what if x is large? For example x = 4539? Most people can't factorise 4539 into primes in their heads. But can a computer do it? Yes, the computer can factorise 4539 in no time. In fact 4539 = 3 × 17 × 89. There is, indeed, an easy way to factorise a number into prime factors. Just apply the method to be described below (using a computer). However, that method is too slow for large numbers: trying to factorise a number with thousands of digits would take more time than the current age of the universe. But is there a fast way? Or more precisely, is there an efficient way? There may be, but no one has found one yet. Some of the most widely used encryption schemes today (such as RSA) make use of the fact that we can't factorise large numbers into prime factors quickly. If such a method is found, a lot of internet transactions will be rendered unsafe. So if you happen to be the discoverer of such a method, don't be too forthcoming in publishing it, consult your national security agency first! Since computers are very good at doing arithmetic, we can work out all the factors of x by simply instructing the computer to divide x by 2 and then 3 then by 5 then by 7 then by 11 ... and so on, and check whether at any point the result is a whole number. Consider the following three examples of the dividing method in action. Example 1 x = 21 x / 2 = 10.5 not a whole number 6 x / 3 = 7 hence 3 and 7 are the factors of 21. Example 2 x = 153 x / 2 = 76.5 hence 2 is not a factor of 153 x / 3 = 51 hence 3 and 51 are factors of 153 51 / 3 = 17 hence 3 and 17 are factors of 153 It is clear that 3, 9, 17 and 51 are the factors of 153. The prime factors of 153 are 3, 3 and 17 (3×3×17 = 153) Example 3 2057 / 2 = 1028.5 ... 2057 / 11 = 187 187 / 11 = 17 hence 11, 11 and 17 are the prime factors of 2057. Exercise Factor the following numbers: 1. 2. 3. 4. 5. 6. 7. 13 26 59 82 101 121 2187 Give up if it takes too long. There is a quick way. Fun Fact -- Is this prime? Interestingly, due to recent developments, we can tell quickly, with the help of a computer program, whether a number is prime with 100% accuracy. 7 2, 5 and 3 The primes 2, 5, and 3 hold a special place in factorisation. Firstly, all even numbers have 2 as one of their prime factors. Secondly, all numbers whose last digit is 0 or 5 can be divided wholly by 5. The third case, where 3 is a prime factor, is the focus of this section. The underlying question is: is there a simple way to decide whether a number has 3 as one of its prime factors? Yes. See the following theorem Theorem - Divisibility by 3 A number is divisible by 3 if and only if the sum of its digits is divisible by 3 E.g. 272 is not divisible by 3, because 2+7+2=11. And 11 isn't divisible by 3. 945 is divisible by 3, because 9+4+5 = 18. And 18 is divisible by 3. In fact 945 / 3 = 315 Is 123456789 divisible by 3? 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = (1 + 9) × 9 / 2 = 45 4+5=9 Nine is divisible by 3, therefore 45 is divisible by 3, therefore 123456789 is divisible by 3! (That this method works is a theorem, though the proof is not given here.) The beauty of the theorem lies in its recursive nature. A number is divisible by 3 if and only if the sum of its digits is divisible by 3. How do we know whether the sum of its digits is divisible by 3? Apply the theorem again! It's too true that "... to recurse, [is] divine". Try a few more numbers yourself. info -- Recursion A prominent computer scientist once said "To iterate is human, to recurse, divine." But what does it mean to recurse? Before that, what is to iterate? "To iterate" simply means doing the same thing over and over again, and computers are very good at that. An example of iteration in mathematics is the exponential operation, e.g. xn means doing x times x times x times x...n times. That is an example of iteration. Thinking about iteration economically (in terms of mental resources), by defining a problem in terms of itself, is "to recurse". To recursively represent xn, we write: xn = 1 if n equals 0. if n > 0 8 What is 99? That is 9 times 9 8. But what is 98, it is 9 times 97. Repeating this way is an example of recursion. Exercises 1. Factorise 1. 2. 3. 45 4050 2187 2. Show that the divisible-by-3 theorem works for any 3 digits numbers (Hint: Express a 3 digit number as 100a + 10b + c, where 0 ≤ a, b and c ≤ 9) 3. "A number is divisible by 9 if and only if the sum of its digits is divisible by 9." True or false? Determine whether 89, 558, 51858, and 41857 are divisible by 9. Check your answers. Finding primes The prime sieve is a relatively efficient method of finding all primes less than or equal to a specified number. Let say we want to find all primes less than or equal to 50. First we write out all numbers between 0 and 51 in a table as below Cross out 1, because it's not a prime. Now 2 is the smallest number not crossed out in the table. We mark 2 as a prime and cross out all multiples of 2 i.e. 4, 6, 8, 10 ... 9 Now 3 is the smallest number not marked in anyway. We mark 3 as a prime and cross out all multiples of 3 i.e. 6, 9, 12, 15 ... Continue is this way to find all the primes. When do you know you have found all the primes under 50? Exercise 1. The prime sieve has been applied to the table above. Notice that every number situated directly below 2 and 5 are crossed out. Construct a rectangular grid of numbers running from 1 to 60 so that after the prime sieve has been performed on it, all numbers situated directly below 3 and 5 are crossed out. What is the width of the grid? 2. Find all primes below 200 Infinitely many primes We know some numbers can be factorised into primes. Some have only themselves as a prime factor, because they are prime. So how many primes are there? There are infinitely many! Here is a classical proof of the infinitude of primes dating back 2000 years to the ancient Greek mathematician Euclid: 10 Proof of infinitude of primes Let us first assume that there are a finite number of primes therefore there must be one prime that is greater than all others, let this prime be referred to as n. We now proceed to show the two assumptions made above will lead to non-sense, and so there are infinitely many primes. Take the product of all prime numbers to yield a number x. Thus: Then, let y equal one more than x: One may now conclude that y is not divisible by any of the primes up to n, since y differs from a multiple of each such prime by exactly 1. Since y is not divisible by any prime number, y must either be prime, or its prime factors must all be greater than n, a contradiction of the original assumption that n is the largest prime! Therefore, one must declare the original assumption incorrect, and that there exists an infinite number of primes. Fun Fact -- Largest known prime The largest prime known to human is 230402457-1. It has a whopping 9152052 digits! Primes of the form 2n-1 are called Mersenne primes named after the French monk/amateur mathematician. Primes in arithmetic progression Consider the arithmetic progression a, a + b, a + 2b, a + 3b .... if a and b share a common factor greater than 1, then a + kb for any k is not prime. But if a and b are coprimes, then there are infinitely many k's such that a + kb is prime! For example, consider a = 3, b = 4, 3, 7, 11, 15, 19, 23, 27, 31... in this rather short list, 3, 7, 11, 19, 23 and 31 are prime and they are all equal to 4k + 3 for some k. And there are infinitely many primes in this progression (see: proof by contradiction exercise Mathematical Proofs). 11 Modular Arithmetic Introduction Modular arithmetic connects with primes in an interesting way. Firstly recall clock-arithmetic. It is a system by which all numbers up to some positive integer, n say, are used. So if you were to start counting you would go 0, 1, 2, 3, ... , n - 1 but instead of counting n you would start over at 0. And what would have been n + 1 would be 1 and what would have been n + 2 would be 2. Once 2n had been reach the number is reset to 0 again, and so on. Very much like the clocks we have which starts at 1 and continues to 12 then back to 1 again. The sequence also continues into what would be the negitive numbers. What would have been 1 is now n - 1. For example, let's start with modulo 7 arithmetic, it's just like ordinary arithmetic except the only numbers we use are 0, 1, 2, 3, 4, 5 and 6. If we see a number outside of this range we add 7 to (or subtract 7 from) it, until it lies within that range. As mentioned above, modular arithmetic is not much different to ordinary arithmetic. For example, consider modulo 7 arithmetic The same deal for multiplication We have done some calculation with negative numbers. Consider 5 × -6. Since -6 does not lie in the range 0 to 6, we need to add 7 to it until it does. And -6 + 7 = 1. So in modular 7 arithmetic, -6 = 1. In the above example we showed that 5 × -6 = -30 = 5, but 5 × 1 = 5. So we didn't do ourselves any harm by using -6 instead of 1. Why? Note - Negatives: The preferred representation of -3 is 4, as -3 + 7 = 4, but using either -3 and 4 in a calculation will give us the same answer as long as we convert the final answer to a number between 0 and 6 (inclusive). Exercise Find in modulo 11 1. -1 × -5 2. 12 3×7 3. Compute all the powers of 2 21, 22, 23, ... , 210 What do you notice? Using the powers of 2 find 61, 62, 63, ... , 610 What do you notice again? 4. i.e. find, by trial and error (or otherwise), all numbers x such that x2 = 4 (mod 11). There are two solutions, find both . 5. i.e. find all numbers x such that x2 = 9 (mod 11). There are two solutions, find both. Inverses Let's consider a number n, the inverse of n is the number that when multiplied by n will give 1. Let's consider a simple example, we want to solve the following equation in modulo 7, the (mod 7) is used to make clear that we doing arithemetic modulo 7. We want to get rid of the 5 somehow, notice that because 3 multiplied by 5 gives 1, so we say 3 is the inverse of 5 in modulo 7. Now we multiply both sides by 3 So x = 2 modulo 7 is the required solution. Inverse is unique 13 From above, we know the inverse of 5 is 3, but does 5 have another inverse? The answer is no. In fact, in any reasonable number system, a number can have one and only one inverse. We can see that from the following proof Suppose n has two inverses b and c From the above argument, all inverses of n must be equal. As a result, if the number n has an inverse, the inverse must be unique. From now on, we will use x-1 to denote the inverse of x if it exists. An interesting property of any modulo n arithmetic is that the number n - 1 has itself as an inverse. That is, (n - 1) × (n - 1) = 1 (mod n), or we can write (n - 1)2 = (-1)2 = 1 (mod n). The proof is left as an exercise at the end of the section. Existence of inverse Not every number has an inverse in every modulo arithmetic. For example, 3 doesn't have an inverse mod 6, i.e., we can't find a number x such that 3x = 1 mod 6 (the reader can easily check). Let's consider modulo 15 arithmetic and note that 15 is composite. We know the inverse of 1 is 1 and of 14 is 14. But what about 3, 6, 9, 12, 5 and 10? None of them has an inverse! Note that each of them shares a common factor with 15! Let's look at 3, we want to use a proof by contradiction argument to show that 3 does not have an inverse modulo 15. Suppose 3 has an inverse, which is denoted by x. We make the jump from modular arithemetic into rational number arithmetic. If 3x = 1 in modulo 15 arithmetic, then for some integer k. Now we divide both sides by 3, we get But this is can't be true, because we know that x is an integer, not a fraction. Therefore 3 doesn't have an inverse in mod 15 arithmetic. To show that 10 doesn't have an inverse is harder and is left as an exercise. We will now state the theorem regarding the existence of inverses in modular arithmetic. Theorem 14 If n is prime then every number (except 0) has an inverse in modulo n arithmetic. Similarly If n is composite then every number that doesn't share a common factor with n has an inverse. It is interesting to note that division is closely related to the concept of inverses. Consider the following expresion the conventional way to calculate the above would be to find the inverse of 3 (being 5). So Let's write the inverse of 3 as 1/3, so we think of multiplying 3-1 as dividing by 3, we get Notice that we got the same answer! In fact, the division method will always work if the inverse exists. Be very careful though, the expression in a different modulo system will produce the wrong answer, for example we don't get 2, as 3-1 does not exist in modulo 9, so we can't use the division method. Exercise 1. Does 8 have an inverse in mod 16 arithemetic? If not, why not? 2. Find x mod 7 if x exists: 3. Calculate x in two ways, finding inverse and division 15 4. (Trick) Find x 5. Find all inverses mod n (n ≤ 19) This exercise may seem tedious, but it will increase your understanding of the topic by tenfold Coprime and greatest common divisor Two numbers are said to be coprimes if their greatest common divisor is 1. The greatest common divisor (gcd) is just what its name says it is. And there is a quick and elegant way to compute the gcd of two numbers, called Euclid's algorithm. Let's illustrate with a few examples: 16 Example 1: Find the gcd of 21 and 49. We set up a 2-column table where the bigger of the two numbers is on the right hand side as follows smaller 21 larger 49 We now compute 49(mod 21) which is 7 and put it in the second row smaller column, and put 21 into the larger column. smaller 21 7 larger 49 21 Perform the same action on the second row to produce the third row. smaller 21 7 0 larger 49 21 7 Whenever we see the number 0 appear on the smaller column, we know the corresponding larger number is the the gcd of the two numbers we started with, i.e. 7 is the gcd of 21 and 49. This algorithm is called Euclid's algorithm. Example 2 Find the gcd of 31 and 101 smaller 31 8 7 1 larger 101 31 8 7 17 0 Example 3 1 Find the gcd of 132 and 200 smaller 132 68 64 4 0 Remember 1. 2. The gcd need not be a prime number. The gcd of two primes is 1 Why does the Euclid's algorithm work? It is more fun for the questioner to discover. Exercise 1. Determine whether the following sets of numbers are coprimes 1. 2. 3. 4. 5050 5051 59 78 111 369 2021 4032 larger 200 132 68 64 4 2. Find the gcd of the numbers 15, 510 and 375 info -- Algorithm An algorithm is a step-by-step description of a series of actions when performed correctly can accomplish a task. There are algorithms for finding primes, deciding whether 2 numbers are coprimes, finding inverses and many other purposes. You'll learn how to implement some of the algorithms we have seen using a computer in the chapter Mathematical Programming. 18 Finding Inverses Let's look at the idea of inverse again, but from a different angle. In fact we will provide a surefire method to find the inverse of any number. Let's consider: 5x = 1 (mod 7) We know x is the inverse of 5 and we can work out it's 3 pretty quickly. But x = 10 is also a solution, so is x = 17, 24, 31, ... 7n + 3. So there are infinitely many solutions; therefore we say 3 is equivalent to 10, 17, 24, 31 and so on. This is a crucial observation Now let's consider A new notation is introduced here, it is the equal sign with three strokes instead of two. It is the "equivalent" sign; the above statement should read "216x is EQUIVALENT to 1" instead of "216x is EQUAL to 1". From now on, we will use the equivalent sign for modulo arithmetic and the equal sign for ordinary arithmetic. Back to the example, we know that x exists, as gcd(811,216) = 1. The problem with the above question is that there is no quick way to decide the value of x! The best way we know is to multiply 216 by 1, 2, 3, 4... until we get the answer, there are at most 816 calculations, way too tedious for humans. But there is a better way, and we have touched on it quite a few times! We notice that we could make the jump just like before into rational mathematics: We jump into rational maths again We jump once more Now the pattern is clear, we shall start from the beginning so that the process is not broken: 19 Now all we have to do is choose a value for f and substitute it back to find a! Remember a is the inverse of 216 mod 811. We choose f = 0, therefore e = 1, d = 13, c = 40, b = 53 and finally a = 199! If choose f to be 1 we will get a different value for a. The very perceptive reader should have noticed that this is just Euclid's gcd algorithm in reverse. Here are a few more examples of this ingenious method in action: Example 1 Find the smallest positive value of a: Choose d = 0, therefore a = 49. Example 2 Find the smallest positive value of a: Choose e = 0, therefore a = -152 = 669 Example 3 Find the smallest positive value of a: 20 Set i = 0, then a = -21 = 34. Why is this so slow for two numbers that are so small? What can you say about the coefficients? 21 Example 4 Find the smallest positive value of a: Now d is not an integer, therefore 21 does not have an inverse mod 102. What we have discussed so far is the method of finding integer solutions to equations of the form: ax + by = 1 where x and y are the unknowns and a and b are two given constants, these equations are called linear Diophantine equations. It is interesting to note that sometimes there is no solution, but if a solution exists, it implies that infinitely many solutions exist. Diophantine equation In the Modular Arithmetic section, we stated a theorem that says if gcd(a,m) = 1 then a-1 (the inverse of a) exists in mod m. It is not difficult to see that if p is prime then gcd(b,p) = 1 for all b less than p, therefore we can say that in mod p, every number except 0 has an inverse. We also showed a way to find the inverse of any element mod p. In fact, finding the inverse of a number in modular arithmetic amounts to solving a type of equations called Diophantine equations. A Diophantine equation is an equation of the form ax + by = d where x and y are unknown. As an example, we should try to find the inverse of 216 in mod 811. Let the inverse of 216 be x, we can write we can rewrite the above in every day arithmetic which is in the form of a Diophantine equation. Now we are going to do the inelegant method of solving the above problem, and then the elegant method (using Magic Tables). 22 Both methods mentioned above uses the Euclid's algorithm for finding the gcd of two numbers. In fact, the gcd is closely related to the idea of an inverse. Let's apply the Euclid's algorithm on the two numbers 216 and 811. This time, however, we should store more details; more specifically, we want to set up an additional column called PQ which stands for partial quotient. The partial quotient is just a technical term for "how many n goes into m" e.g. The partial quotient of 3 and 19 is 6, the partial quotient of 4 and 21 is 5 and one last example the partial quotient of 7 and 49 is 7. smaller 216 163 The tables says three 216s goes into 811 with remainder 163, or symbollically: 811 = 3×216 + 163. Let's continue: smaller 216 163 53 4 1 0 larger 811 216 163 53 4 1 PQ 3 1 3 13 4 larger 811 PQ 3 Reading off the table, we can form the following expressions 811 = 3× 216 + 163 216 = 1× 163 + 53 163 = 3× 53 + 4 53 =13× 4 + 1 Now that we can work out the inverse of 216 by working the results backwards 1 = 53 - 13×4 1 = 53 - 13×(163 - 3×53) 23 1 = 40×53 - 13×163 1 = 40×(216 - 163) - 13×163 1 = 40×216 - 53×163 1 = 40×216 - 53×(811 - 3×216) 1 = 199×216 - 53×811 Now look at the equation mod 811, we will see the inverse of 216 is 199. Magic Table The Magic Table is a more elegant way to do the above calculations, let us use the table we form from Euclid's algorithm smaller 216 163 53 4 1 0 larger 811 216 163 53 4 1 PQ 3 1 3 13 4 24 Now we set up the so-called "magic table" which looks like this initially 0 1 1 0 Now we write the partial quotient on the first row: 3 1 0 1 1 0 3 1 3 4 We produce the table according to the following rule: Multiply a partial quotient one space to the left of it in a different row, add the product to the number two space to the left on the same row and put the sum in the corresponding row. It sounds more complicated then it should. Let's illustrate by producing a column: 3 1 0 1 1 0 3 1 3 1 3 4 We put a 3 in the second row because 3 = 3×1 + 0. We put a 1 in the third row because 1 = 3×0 + 1. We shall now produce the whole table without disruption: 3 1 0 1 1 0 3 4 1 1 3 1 5 4 13 19 9 53 4 81 1 21 6 I claim |199×216 - 811×53| = 1 In fact, if you have done the magic table properly and cross multiplied and subtracted the last 25 two column correctly, then you will always get 1 or -1, provided the two numbers you started with are coprimes. The magic table is just a cleaner way of doing the mathematics. 26 Exercises 1. Find the smallest positive x: 2. Find the smallest positive x: 3. (a) Produce the magic table for 33a = 1 (mod 101) (b) Evaluate and express in the form p/q What do yo notice? 4. (a) Produce the magic table for 17a = 1 (mod 317) (b) Evaluate and express in the form p/q What do yo notice? Chinese remainder theorem The Chinese remainder theorem is known in China as Han Xing Dian Bing, which in its most naive translation means Han Xing counts his soldiers. The original problem goes like this: There exists a number x, when divided by 3 leaves remainder 2, when divided by 5 leaves remainder 3 and when divided by 7 leaves remaider 2. Find the smallest x. We translate the question into symbolic form: 27 How do we go about finding such a x? We shall use a familar method and it is best illustrated by example: Looking at x = 2 (mod 3), we make the jump into ordinary mathematics Now we look at the equation modulo 5 Substitute into (1) to get the following Now look at the above modulo 7 we get We choose b = 1 to minimise x, therefore x = 23. And a simple check (to be performed by the reader) should confirm that x = 23 is a solution. A good question to ask is what is the next smallest x that satisfies the three congruences? The answer is x = 128, and the next is 233 and the next is 338, and they differ by 105, the product of 3, 5 and 7. We will illustrate the method of solving a system of congruencies further by the following examples: Example 1 Find the smallest x that satifies: 28 Solution now substitute back into the first equation, we get we obtain again substituting back Therefore 52 is the smallest x that satifies the congruencies. Example 2 Find the smallest x that satisfies: Solution 29 substituting back now solve for b again, substitue back Therefore 269 is the smallest x that satifies the Congruences. Excercises 1. Solve for x 2. Solve for x *Existence of a solution* The exercises above all have a solution. So does there exist a system of congruences such that no solution could be found? It certainly is possible, consider: x ≡ 5 (mod 15) x ≡ 10 (mod 21) 30 a cheekier example is: x ≡ 1 (mod 2) x ≡ 0 (mod 2) but we won't consider silly examples like that. Back to the first example, we can try to solve it by doing: the above equation has no solution because 3 does not have an inverse modulo 21! One may be quick to conclude that if two modulo systems share a common factor then there is no solution. But this is not true! Consider: we can find a solution we now multiply both sides by the inverse of 5 (which is 17), we obtain obviously, k = 3 is a solution, and the two modulo systems are the same as the first example (i.e. 15 and 21). So what determines whether a system of congruences has a solution or not? Let's consider the general case: we have 31 essentially, the problem asks us to find k and l such that the above equations are satisfied. We can approach the problem as follows now suppose m and n have gcd(m,n) = d, and m = dmo, n = dno. We have if (a - b)/d is an integer then we can read the equation mod mo, we have: Again, the above only makes sense if (a - b)/d is integeral. Also if (a - b)/d is an integer, then there is a solution, as mo and no are coprimes! In summary: for a system of two congruent equations there is a solution if and only if d = gcd(m,n) divides (a - b) And the above generalises well into more than 2 congruences. For a system of n congruences: ... for a solution to exist, we require that if i ≠ j gcd(mi,mj) divides (ai - aj) 32 Exercises Decide whether a solution exists for each of the congruencies. Explain why. 1. x ≡ 7 (mod 25) x ≡ 22 (mod 45) 2. x ≡ 7 (mod 23) x ≡ 3 (mod 11) x ≡ 3 (mod 13) 3. x ≡ 7 (mod 25) x ≡ 22 (mod 45) x ≡ 7 (mod 11) 4. x ≡ 4 (mod 28) x ≡ 28 (mod 52) x ≡ 24 (mod 32) To go further This chapter has been a gentle introduction to number theory, a profoundly beautiful branch of mathematics. It is gentle in the sense that it is mathematically light and overall quite easy. If you enjoyed the material in this chapter, you would also enjoy Further Modular Arithmetic, which is a harder and more rigorous treatment of the subject. Also, if you feel like a challenge you may like to try out the Probelem Set we have prepared for you. On the other hand, the project asks you to take a more investigative approach to work through some of the finer implications of the Chinese Remainder Theorem. Acknowledgement Acknowledgement: This chapter of the textbook owes much of its inspiration to Terry Gagen, Emeritus Associate Professor of Mathematics at the University of Sydney, and his lecture notes on "Number Theory and Algebra". Terry is a much loved figure among his students and is 33 renowned for his entertaining style of teaching. Reference 1. Largest Known Primes--A Summary Feedback What do you think? Too easy or too hard? Too much information or not enough? How can we improve? Please let us know by leaving a comment in the discussion section. Better still, edit it yourself and make it better. Problem Set 1. Is there a rule to determine whether a 3-digit number is divisible by 11? If yes, derive that rule. 2. Show that p, p + 2 and p + 4 cannot all be primes. (p a positive integer) 3. Find x 4. Show that there are no integers x and y such that 5. In modular arithmetic, if for some m, then we can write we say, x is the square root of y mod m. Note that if x satisfies x2 ≡ y, then m - x ≡ -x when squared is also equivalent to y. We consider both x and -x to be square roots of y. Let p be a prime number. Show that (a) where 34 E.g. 3! = 1*2*3 = 6 35 (b) Hence, show that for p ≡ 1 (mod 4), i.e., show that the above when squared gives one. 36 Square root of minus 1 Project -- The Square Root of -1 Notation: In modular arithmetic, if for some m, then we can write we say, x is the square root of y mod m. Note that if x satisfies x2 ≡ y, then m - x ≡ -x when squared is also equivalent to y. We consider both x and -x to be square roots of y. 1. Question 5 of the Problem Set showed that exists for p ≡ 1 (mod 4) prime. Explain why no square root of -1 exist if p ≡ 3 (mod 4) prime. 2. Show that for p ≡ 1 (mod 4) prime, there are exactly 2 solutions to 3. Suppose m and n are integers with gcd(n,m) = 1. Show that for each of the numbers 0, 1, 2, 3, .... , nm - 1 there is a unique pair of numbers a and b such that the smallest number x that satisfies: x ≡ a (mod m) x ≡ b (mod n) is that number. E.g. Suppose m = 2, n = 3, then 4 is uniquely represented by x ≡ 0 (mod 2) x ≡ 1 (mod 3) as the smallest x that satisfies the above two congruencies is 4. In this case the unique pair of numbers are 0 and 1. 4. If p ≡ 1 (mod 4) prime and q ≡ 3 (mod 4) prime. Does 37 have a solution? Why? 5. If p ≡ 1 (mod 4) prime and q ≡ 1 (mod 4) prime and p ≠ q. Show that has 4 solutions. 6. Find the 4 solutions to note that 493 = 17 × 29. 7. Take an integer n with more than 2 prime factors. Consider: Under what condition is there a solution? Explain thoroughly. 38 Solutions to exercises HSE Primes|Primes and Modular Arithmetic At the moment, the main focus is on authoring the main content of each chapter. Therefore this exercise solutions section may be out of date and appear disorganised. If you have a question please leave a comment in the "discussion section" or contact the author or any of the major contributors. Factorisation Exercises Factorise the following numbers. (note: I know you didn't have to, this is just for those who are curious) 1. 2. 3. 4. 5. 6. 7. 101 is prime 59 is prime 13 is prime Recursive Factorisation Exercises Factorise using recursion. 1. 2. 3. 39 Prime Sieve Exercises 1. Use the above result to quickly work out the numbers that still need to be crossed out in the table below, knowing 5 is the next prime: The next prime number is 5. Because 5 is an unmarked prime number, and 5 * 5 = 25, cross out 25. Also, 7 is an unmarked prime number, and 5 * 7 = 35, so cross off 35. However, 5 * 11 = 55, which is too high, so mark 5 as prime ad move on to 7. The only number low enough to be marked off is 7 * 7, which equals 35. You can go no higher. 2. Find all primes below 200. The method will not be outlined here, as it is too long. However, all primes below 200 are: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 Modular Arithmetic Exercises 1. +5=5 2. 3. 21 = 2,22 = 4,23 = 8,24 = 16 = 5 2 = 32 = 10,26 = 64 = 9,27 = 128 = 7 28 = 256 = 3,29 = 512 = 6,210 = 1024 = 1 An easier list: 2, 4, 8, 5, 10, 9, 7, 3, 6, 1 Notice that it is not necessary to acutally compute 210 to find 210 mod 11. If you know 29 mod 11 = 6. You can find 210 mod 11 = (2*(29 mod 11)) mod 11 = 2*6 mod 11 = 12 mod 11 = 1. We can note that 29 = 6 and 210 = 1, we can calculate 62 easily: 62 = 218 = 2^8 = 3. OR by the above method 61 = 6,62 = 36 = 3,63 = 6 * 3 = 18 = 7, 64 = 6 * 7 = 42 = 9,65 = 6 * 9 = 54 = 10,66 = 6 * 10 = 60 = 5, 67 = 6 * 5 = 30 = 8,68 = 6 * 8 = 48 = 4,69 = 6 * 4 = 24 = 2,610 = 6 * 2 = 12 = 1. An easier list: 6, 3, 7, 9, 10, 5, 8, 4, 2, 1. 5 alternatively, -1 = 10, -5 = 6: 10 × 6 = 60 = 5× 11 40 4. 02 = 0, 12 = 1, 22 = 4, 32 = 9, 4 = 16 = 5, 52 = 25 = 5, 62 = 36 = 3, 72 = 49 = 3, 82 = 64 = 5, 92 = 81 = 4, 102 = 100 = 1 An easier list: 0, 1, 4, 9, 5, 3, 3, 5, 9, 4, 1 2 Thus 5. x2 = -2 = 9 Just look at the list above and you'll see that Division and Inverses Exercises 1. x = 2 -1 = 4 x = 3 -1 = 5 x = 4 -1 = 2 x = 5 -1 = 3 x = 6 -1 = 6 x = 7 - 1 = 0 - 1 therefore the inverse does not exist 2. 3. 41 4. 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 13 5 0 8 11 9 6 7 9 5 4 13 13 7 11 4 3 5 7 6 4 3 9 5 10 8 3 13 2 7 5 13 16 11 12 17 2 9 15 2 2 4 5 3 7 2 2 3 3 2 4 5 3 5 2 6 7 4 3 8 7 11 2 5 3 11 4 7 8 9 5 10 11 6 9 11 3 12 13 7 5 14 15 11 8 16 17 15 14 6 9 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 mod 2 mod 3 mod 4 mod 5 mod 6 mod 7 mod 8 mod 9 mod 10 mod 11 mod 12 mod 13 mod 14 mod 15 mod 16 mod 17 mod 18 18 mod 19 12 14 10 4 5 7 8 7 3 42 Coprime and greatest common divisor Exercises 1. 1. smaller 5050 1 0 larger 5051 5050 1 5050 and 5051 are coprime 2. smaller 59 19 2 1 0 larger 78 59 19 2 1 59 and 79 are coprime 43 3. smaller 111 36 3 0 larger 369 111 36 3 111 and 369 are not coprime 4. smaller 2021 2011 10 1 0 larger 4032 2021 2011 10 1 2021 and 4032 are coprime 44 2.We first calculate the gcd for all combinations smaller 15 0 larger 510 15 smaller 15 0 larger 375 15 smaller 375 135 105 30 15 0 larger 510 375 135 105 30 15 The gcd for any combination of the numbers is 15 so the gcd is 15 for the three numbers. Diophantine equation Exercises 1. There is no solution, because can never become an integer. 45 2. We choose d=1, then x=26. 3. (a) smaller 33 2 1 0 larger 101 33 2 1 P Q 3 1 6 2 3 0 1 1 0 3 1 16 49 16 2 101 33 (b) To be added 46 4. (a) smaller 17 11 6 5 1 0 larger 317 17 11 6 5 1 PQ 18 1 1 1 5 18 0 1 1 0 18 1 1 19 1 1 37 2 1 56 3 5 317 17 (b) To be added 47 Chinese remainder theorem exercises 1. Question 1 Show that the divisible-by-3 theorem works for any 3 digits numbers (Hint: Express a 3 digit number as 100a + 10b + c, where a, b and c are ≥ 0 and < 10) Solution 1 Any 3 digits integer x can be expressed as follows x = 100a + 10b + c where a, b and c are positive integer between 0 and 9 inclusive. Now if and only if a + b + c = 3k for some k. But a, b and c are the digits of x. Question 2 "A number is divisible by 9 if and only if the sum of its digits is divisible by 9." True or false? Determine whether 89, 558, 51858, and 41857 are divisible by 9. Check your answers. Solution 2 The statement is true and can be proven as in question 1. 48 Question 4 The prime sieve has been applied to the table of numbers above. Notice that every number situated directly below 2 and 5 are crossed out. Construct a rectangular grid of numbers running from 1 to 60 so that after the prime sieve has been performed on it, all numbers situated directly below 3 and 5 are crossed out. What is the width of the grid? Solution 4 The width of the grid should be 15 or a multiple of it. Question 6 Show that n - 1 has itself as an inverse modulo n. Solution 6 (n - 1)2 = n2 - 2n + 1 = 1 (mod n) Alternatively (n - 1)2 = (-1)2 = 1 (mod n) Question 7 Show that 10 does not have an inverse modulo 15. Solution 7 Suppose 10 does have an inverse x mod 15, 10x = 1 (mod 15) 2Ã-5x = 1 (mod 15) 5x = 8 (mod 15) 5x = 8 + 15k for some integer k x = 1.6 + 3k but now x is not an integer, therefore 10 does not have an inverse 49 Problem set solutions Question 1 Is there a rule to determine whether a 3-digit number is divisible by 11? If yes, derive that rule. Solution Let x be a 3-digit number We have now We can conclude a 3-digit number is divisible by 11 if and only if the sum of first and last digit minus the second is divisible by 11. Question 2 Show that p, p + 2 and p + 4 cannot all be primes. (p a positive integer) Solution We look at the arithmetic mod 3, then p slotted into one of three categories 1st category we deduce p is not prime, as it's a multiple of 3 2nd category so p + 2 is not prime 3rd category therefore p + 4 is not prime 50 Therefore p, p + 2 and p + 4 cannot all be primes. Question 3 Find x Solution Notice that . Then . Likewise, and . Then Question 4 9. Show that there are no integers x and y such that Solution Look at the equation mod 5, we have 51 but therefore there does not exist a x such that Question 5 Let p be a prime number. Show that (a) where E.g. 3! = 1Ã-2Ã-3 = 6 (b) Hence, show that for p ≡1 (mod 4) Solution a) If p = 2, then it's obvious. So we suppose p is an odd prime. Since p is prime, some deep thought will reveal that every distinct element multiplied by some other element will give 1. Since we can pair up the inverses (two numbers that multiply to give one), and (p - 1) has itself as an inverse, therefore it's the only element not "eliminated" 52 as required. b) From part a) since p = 4k + 1 for some positive integer k, (p - 1)! has 4k terms there are an even number of minuses on the right hand side, so it follows and finally we note that p = 4k + 1, we can conclude 53 Definitions Logic Introduction Logic is the study of the way we humans reason. In this chapter, we focus on the methods of logical reasoning, i.e. digital logic, predicate calculus, application to proofs and the (insanely) fun logical puzzles. Boolean algebra In the black and white world of ideals, there is absolute truth. That is to say everything is either true or false. With this philosophical backdrop, we consider the following examples: "One plus one equals two." True or false? That is (without a doubt) true! "1 + 1 = 2 AND 2 + 2 = 4." True or false? That is also true. But what about: "1 + 1 = 3 OR Sydney is in Australia" True or false? It is true! Although 1 + 1 = 3 is not true, the OR in the statement made it so that if either part of the statement is true then the whole statement is true. Now let's consider a more puzzling example "2 + 2 = 4 OR 1 + 1 = 3 AND 1 - 3 = -1" True or false? The truth or falsity of the statements depends on the order in which you evaluate the statement. If you evaluate "2 + 2 = 4 OR 1 + 1 = 3" first, the statement is false, and otherwise true. As in ordinary algebra, it is necessary that we define some rules to govern the order of evaluation, so we don't have to deal with ambiguity. Before we decide which order to evaluate the statements in, we do what most mathematician love to do -replace sentences with symbols. Let x represent the truth or falsity of the statement 2 + 2 = 4. Let y represent the truth or falsity of the statement 1 + 1 = 3. Let z represent the truth or falsity of the statement 1 - 3 = -1. Then the above example can be rewritten in a more compact way: 54 x OR y AND z To go one step further, mathematicians also replace OR by + and AND by Ã-, the statement becomes: Now that the order of precedence is clear. We evaluate (y AND z) first and then OR it with x. The statement "x + yz" is true, or symbolically x + yz = 1 where the number 1 represents "true". There is a good reason why we choose the multiplicative sign for the AND operation. As we shall see later, we can draw some parallels between the AND operation and multiplication. The Boolean algebra we are about to investigate is named after the British mathematician George Boole. Boolean algebra is about two things -- "true" or "false" which are often represented by the numbers 1 and 0 respectively. Alternative, T and F are also used. Boolean algebra has operations (AND and OR) analogous to the ordinary algebra that we know and love. Basic Truth tables We have all had to memorise the 9 by 9 multiplication table and now we know it off by heart. In Boolean algebra, the idea of a truth table is somewhat similar. Let's consider the AND operation which is analogous to the multiplication. We want to consider: x AND y where and x and y each represent a true or false statement (e.g. It is raining today). It is true if and only if both x and y are true, in table form: The AND function x F F T T y F T F T x AND y F F F T 55 We shall use 1 instead of T and 0 instead of F from now on. The AND function x 0 0 1 1 y 0 1 0 1 x AND y 0 0 0 1 Now you should be able to see why we say AND is analogous to multiplication, we shall replace the AND by Ã-, so x AND y becomes xÃ-y (or just xy). From the AND truth table, we have: 0 Ã- 0 = 0 0 Ã- 1 = 0 1 Ã- 0 = 0 1 Ã- 1 = 1 To the OR operation. x OR y is FALSE if and only if both x and y are false. In table form: The OR function x 0 0 1 1 y 0 1 0 1 x OR y 0 1 1 1 We say OR is almost analoguous to addition. We shall illustrate this by replacing OR with +: 0+0=0 0+1=1 1+0=1 1 + 1 = 1 (like 1 OR 1 is 1) 56 The NOT operation is not a binary operation, like AND and OR, but a unary operation, meaning it works with one argument. NOT x is true if x is false and false if x is true. In table form: The NOT function x 0 1 NOT x 1 0 In symbolic form, NOT x is denoted x' or ~x (or by a bar over the top of x). Alternative notations: and Compound truth tables The three truth tables presented above are the most basic of truth tables and they serve as the building blocks for more complex ones. Suppose we want to construct a truth table for xy + z (i.e. x AND y OR z). Notice this table involves three variables (x, y and z), so we would expect it to be bigger then the previous ones. To construct a truth table, firstly we write down all the possible combinations of the three variables: x 0 0 0 0 1 1 1 y 0 0 1 1 0 0 1 z 0 1 0 1 0 1 0 57 1 1 1 There is a pattern to the way the combinations are written down. We always start with 000 and end with 111. As to the middle part, it is up to the reader to figure out. We then complete the table by hand computing what value each combination is going to produce using the expression xy + z. For example: 000 x = 0, y = 0 and z = 0 xy + z = 0 001 x = 0, y = 0 and z = 1 xy + z = 1 We continue in this way until we fill up the whole table The procedure we follow to produce truth tables are now clear. Here are a few more examples of truth tables. Example 1 -- x + y + z 58 Example 2 -- (x + yz)' When an expression is hard to compute, we can first compute intermediate results and then the final result. Example 3 -- (x + yz')w 59 Exercise Produce the truth tables for the following operations: 8. 9. 10. NAND: x NAND y = NOT (x AND y) NOR: x NOR y = NOT (x OR y) XOR: x XOR y is true if and ONLY if one of x or y is true. Produce truth tables for: 4. 5. 6. 7. 8. xyz x'y'z' xyz + xy'z xz (x + y)' 60 9. 10. 11. x'y' (xy)' x' + y' Laws of Boolean algebra In ordinary algebra, two expressions may be equivalent to each other, e.g. xz + yz = (x + y)z. The same can be said of Boolean algebra. Let's construct truth tables for: xz + yz (x + y)z xz + yz (x + y)z 61 By comparing the two tables, you will have noticed that the outputs (i.e. the last column) of the two tables are the same! Definition We say two Boolean expressions are equivalent if the output of their truth tables are the same. We list a few expressions that are equivalent to each other x+0=x x Ã- 1 = x xz + yz = (x + y)z x + x' = 1 x Ã- x' = 0 x Ã- x = x x + yz = (x + y)(x + z) Take a few moments to think about why each of those laws might be true. The last law is not obvious but we can prove that it's true using the other laws: As Dr Kuo Tzee-Char, Honorary reader of mathematics at the University of Sydney, is so fond of saying: "The only thing to remember in mathematics is that there is nothing to remember. Remember that!". You should not try to commit to memory the laws as they are stated, because some of them are so deadly obvious once you are familiar with the AND, OR and NOT operations. You should only try to remember those things that are most basic, once a high level of familiarity is developed, you will agree there really isn't anything to remember. Simplification Once we have those laws, we will want to simplify Boolean expressiosn just like we do in ordinary algebra. We can all simplify the following example with ease: 62 the same can be said about: From those two examples we can see that complex-looking expressions can be reduced very significantly. Of particular interest are expressions of the form of a sum-of-product, for example: xyz + xyz' + xy'z + x'yz + x'y'z' + x'y'z We can factorise and simplify the expression as follows xyz + xyz' + xy'z + x'yz + x'y'z' + x'y'z It is only hard to go any further, although we can. We use the identity: x + yz = (x + y)(x + z) If the next step is unclear, try constructing truth tables as an aid to understanding. And this is as far as we can go using the algebraic approach (or any other approach). The algebraic approach to simplication relies on the priniciple of elimination. Consider, in ordinary algebra: x+y-x We simplify by rearranging the expression as follows (x - x) + y = y 63 Although we only go through the process in our head, the idea is clear: we bring together terms that cancel themselves out and so the expression is simplified. 64 De Morgan's theorems So far we have only dealt with expressions in the form of a sum of products e.g. xyz + x'z + y'z'. De Morgan's theorems help us to deal with another type of Boolean expressions. We revisit the AND and OR truth tables: x 0 0 0 1 1 1 1 1 1 1 0 0 1 0 y 0 x Ã- y 0 x+y You would be correct to suspect that the two operations are connected somehow due to the similarities between the two tables. In fact, if you invert the AND operation, i.e. you perform the NOT operations on x AND y. The outputs of the two operations are almost the same: x 0 0 0 1 1 1 1 1 1 0 0 1 1 1 y 0 (x Ã- y)' 1 x+y 65 66 The connection between AND, OR and NOT is revealed by reversing the output of x + y by replacing it with x' + y'. x 0 1 0 1 1 1 1 0 1 0 0 1 1 1 y 0 (x Ã- y)' 1 x' + y' Now the two outputs match and so we can equate them: (xy)' = x' + y' this is one of de Morgan's laws. The other which can be derived using a similar process is: (x + y)' = x'y' We can apply those two laws to simplify equations: Example Express x in sum of product form 1 Example Express x in sum of product form 2 This points to a possible extension of De Morgan's laws to 3 or more 67 variables. Example Express x in sum of product form 3 Example Express x in sum of product form 4 Another thing of interest we learnt is that we can reverse the truth table of any expression by replacing each of its variables by their opposites, i.e. replace x by x' and y' by y etc. This result shouldn't have been a suprise at all, try a few examples yourself. De Morgan's laws (x + y)' = x'y' (xy)' = x' + y' Exercise 3. 1. 2. 3. 4. 5. 4. Express in simplified sum-of-product form: z = ab'c' + ab'c + abc z = ab(c + d) z = (a + b)(c + d + f) z = a'c(a'bd)' + a'bc'd' + ab'c z = (a' + b)(a + b + d)d' Show that x + yz is equivalent to (x + y)(x + z) 68 Propositions We have been dealing with propositions since the start of this chapter, although we are not told they are propositions. A proposition is simply a statement (or sentence) that is either TRUE or FALSE. Hence, we can use Boolean algebra to handle propositions. There are two special types of propositions -- tautology and contradiction. A tautology is a proposition that is always TRUE, e.g. "1 + 1 = 2". A contradiction is the opposite of a tautology, it is a proposition that is always FALSE, e.g. 1 + 1 = 3. As usual, we use 1 to represent TRUE and 0 to represent FALSE. Please note that opinions are not propositions, e.g. "George W. Bush started the war on Iraq for its oil." is just an opinion, its truth or falsity is not universal, meaning some think it's true, some do not. Examples • • • • • • • "It is raining today" is a proposition. "Sydney is in Australia" is a proposition. "1 + 2 + 3 + 4 + 5 = 16" is a proposition. "Earth is a perfect sphere" is a proposition. "How do you do?" is not a proposition - it's a question. "Go clean your room!" is not a proposition - it's a command. "Martians exist" is a proposition. Since each proposition can only take two values (TRUE or FALSE), we can represent each by a variable and decide whether compound propositions are true by using Boolean algebra, just like we have been doing. For example "It is always hot in Antarctica OR 1 + 1 = 2" will be evaluated as true. Implications Propositions of the type if something something then something something are called implications. The logic of implications are widely applicable in mathematics, computer science and general everyday common sense reasoning! Let's start with a simple example "If 1 + 1 = 2 then 2 - 1 = 1" is an example of implication, it simply says that 2 - 1 = 1 is a consequence of 1 + 1 = 2. It's like a cause and effect relationship. Consider this example: John says: "If I become a millionaire, then I will donate $500,000 to the Red Cross." 69 There are four situations: 5. 6. 7. 8. John becomes a millionaire and donates $500,000 to the Red Cross John becomes a millionaire and does not donate $500,000 to the Red Cross John does not become a millionaire and donates $500,000 to the Red Cross John does not become a millionaire and does not donate $500,000 to the Red Cross In which of the four situations did John NOT fulfill his promise? Clearly, if and only if the second situation occured. So, we say the proposition is FALSE if and only if John becomes a millionaire and does not donate. If John did not become a millionaire then he can't break his promise, because his promise is now claiming nothing, therefore it must be evaluated TRUE. If x and y are two propositions, x implies y (if x then y), or symbolically has the following truth table: x 0 0 1 1 y 0 1 0 1 1 1 0 1 For emphasis, is FALSE if and only if x is true and y false. If x is FALSE, it does not matter what value y takes, the proposition is automatically TRUE. On a side note, the two propositions x and y need not have anything to do with each other, e.g. "1 + 1 = 2 implies Australia is in the southern hemisphere" evaluates to TRUE! If then we express it symbolically as . 70 It is a two way implication which translates to x is TRUE if and only if y is true. The if and only if operation has the following truth table: x 0 0 1 1 y 0 1 0 1 1 0 0 1 The two new operations we have introduced are not really new, they are just combinations of AND, OR and NOT. For example: Check it with a truth table. Because we can express the implication operations in terms of AND, OR and NOT, we have open them to manipulation by Boolean algebra and de Morgan's laws. Example Is the following proposition a tautology (a proposition that's always true) 1 Solution 1 Therefore it's a tautology. Solution 2 A somewhat easier solution is to draw up a truth table of the proposition, and note that the output column are all 1s. Therefore the proposition is a tautology, because the output is 1 regardless of the inputs (i.e. x, y and z). 71 Example Show that the proposition z is a contradiction (a proposition that is always false): z = xy(x + y)' Solution 2 Therefore it's a contradiction. Back to Example 1, : . This isn't just a slab of symbols, you should be able translate it into everyday language and understand intuitively why it's true. Exercises 8. 1. 2. 9. 1. Decide whether the following propositions are true or false: If 1 + 2 = 3, then 2 + 2 = 5 If 1 + 1 = 3, then fish can't swim Show that the following pair of propositions are equivalent : Logic Puzzles Puzzle is an all-encompassing word, it refers to anything trivial that requires solving. Here is a collection of logic puzzles that we can solve using Boolean algebra. Example 1 We have two type of people -- knights or knaves. A knight always tell the truth but the knaves always lie. Two people, Alex and Barbara, are chatting. Alex says :"We are both knaves" Who is who? We can probably work out that Alex is a knave in our heads, but the algebraic approach to determine Alex 's identity is as follows: Let A be TRUE if Alex is a knight 72 Let B be TRUE if Barbara is a knight There are two situations, either: Alex is a knight and what he says is TRUE, OR he is NOT a knight and what he says it FALSE. There we have it, we only need to translate it into symbols: A(A'B') + A'[(A'B')'] = 1 we simplify: (AA')B' + A'[A + B] = 1 A'A + A'B = 1 A'B = 1 Therefore A is FALSE and B is TRUE. Therefore Alex is a knave and Barbara a knight. Example 2 There are three businessmen, conveniently named Abner, Bill and Charley, who order martinis together every weekend according to the following rules: 4. 5. 6. 7. 2. 3. 4. 5. If A orders a martini, so does B. Either B or C always order a martini, but never at the same lunch. Either A or C always order a martini (or both) If C orders a martini, so does A. or AB + A'B' = 1 B'C + BC' = 1 A+C=1 or CA + C'A' = 1 Putting all these into one formula and simplifying: 73 Exercises Please go to Logic puzzles. Problem Set 1. Decide whether the following propositions are equivalent: 2. Express in simplest sum-of-product form the following proposition: 3. Translate the following sentences into symbolic form and decide if it's true: a. For all x, if x2 = 9 then x2 - 6x - 3 = 0 b. We can find a x, such that x2 = 9 and x2 - 6x - 3 = 0 are both true. 4. NAND is a binary operation: x NAND y = (xy)' Find a proposition that consists of only NAND operators, equivalent to: (x + y)w + z 5. Do the same with NOR operators. Recall that x NOR y = (x + y)' 74 Feedback What do you think? Too easy or too hard? Too much information or not enough? How can we improve? Please let us know by leaving a comment in the discussion section. Better still, edit it yourself and make it better. 75 Mathematical proofs "It is by logic that we prove, but by intuition that we discover." Introduction Mathematicians have been, for the past five hundred years or so, obsessed with proofs. They want to prove everything, and in the process proved that they can't prove everything (see this). This chapter will introduce the techniques of mathematical induction, proof by contradiction and the axiomatic approach to mathematics. Mathematical induction Deductive reasoning is the process of reaching a conclusion that is guaranteed to follow. For example, if we know • • All ravens are black birds, and For every action, there is an equal and opposite reaction then we can conclude: • • This bird is a raven, therefore it is black. This billiard ball will move when struck with a cue. Induction is the opposite of deduction. To induce, we observe how things behave in specific cases and from that we draw conclusions as to how things behave in the general case. For example: We know it is true for all numbers, because Gauss told us. But how do we show that it's true for all positive integers? Even if we can show the identity holds for numbers from one to a trillion or any larger number we can think of, we still haven't proved that it's true for all positive integers. This is where mathematical induction comes in, it works somewhat like the dominoes. If we can show that the identity holds for some number k, and that mere fact implies that the identity also holds for k + 1, then we have effectively shown that it works for all integers. Example 1 Show that the identity 76 holds for all positive integers. Solution Firstly, we show that it holds for integers 1, 2 and 3 1 = 2Ã-1/2 1 + 2 = 3Ã-2/2 1 + 2 + 3 = 4Ã-3/2 = 6 Suppose the identity holds for some number k: This supposition is known as the induction hypothesis. We assume it is true, and aim to show that, is also true. We proceed which is what we have set out to show. Since the identity holds for 3, it also holds for 4, and since it holds for 4 it also holds for 5, and 6, and 7, and so on. There are two types of mathematical induction: strong and weak. In weak induction, you assume the identity holds for certain value k, and prove it for k+1. In strong induction, the identity must be true for any value lesser or equal to k, and then prove it for k+1. 77 Example 2 Show that n! > 2n for n ≥ 4. Solution The claim is true for n = 4. As 4! > 24, i.e. 24 > 16. Now suppose it's true for n = k, k ≥ 4, i.e. k! > 2k it follows that (k+1)k! > (k+1)2k > 2k+1 (k+1)! > 2k+1 We have shown that if for n = k then it's also true for n = k + 1. Since it's true for n = 4, it's true for n = 5, 6, 7, 8 and so on for all n. Example 3 Show that Solution Suppose it's true for n = k, i.e. it follows that We have shown that if it's true for n = k then it's also true for n = k + 1. Now it's true for n = 1 (clear). Therefore it's true for all integers. Exercises 1. Prove that 2. Prove that for n ≥ 1, 78 where xn and yn are integers. 3. Note that Prove that there exists an explicit formula for for all integer m. E.g. 4. The sum of all of the interior angles of a triangle is ; the sum of all the angles of a rectangle is . Prove that the sum of all the angles of a polygon with n sides, is . Proof by contradiction "When you have eliminated the impossible, what ever remains, however improbable must be the truth." Sir Arthur Conan Doyle The idea of a proof by contradiction is to: 6. 7. 8. First, we assume that the opposite of what we wish to prove is true. Then, we show that the logical consequences of the assumption include a contradiction. Finally, we conclude that the assumption must have been false. Root 2 is irrational As an example, we shall prove that is not a rational number. Recall that a rational number is a number which can be expressed in the form of p/q, where p and q are integers and q does not equal 0 (see the 'categorizing numbers' section here). First, assume that is rational: where a and b are coprimes (i.e. both integers with no common factors). If a and b are not coprimes, we remove all common factors. In other words, a/b is in simplest form. Now, 79 continuing: We have now found that a2 is some integer multiplied by 2. Therefore, a2 must be divisible by two. If a2 is even, then a must also be even, for an odd number squared yields an odd number. Therefore we can write a = 2c, where c is another integer. We have discovered that b2 is also an integer multiplied by two. It follows that b must be even. We have a contradiction! Both a and b are even integers. In other words, both have the common factor of 2. But we already said that a/b is in simplest form. Since such a contradiction has been established, we must conclude that our original assumption was false. Therefore, âˆš2 is irrational. Contrapositive Some propositions that take the form of if xxx then yyy can be hard to prove. It is sometimes useful to consider the contrapositive of the statement. Before I explain what contrapositive is let us see an example "If x2 is odd then x is is also odd" is harder to prove than "if x even then x2 is also even" although they mean the same thing. So instead of proving the first proposition directly, we prove the second proposition instead. If A and B are two propositions, and we aim to prove If A is true then B is true we may prove the equivalent statement If B is false then A is false instead. This technique is called proof by contrapositive. To see why those two statements are equivalent, we show the following boolean algebra 80 expressions is true (see Logic) (to be done by the reader). Exercises 1.Prove that there is no perfect square number for 11,111,1111,11111...... 2. Prove that there are infinitely number of k's such that, 4k + 3, is prime. (Hint: consider N = p1p2...pm + 3) Reading higher mathematics This is some basic information to help with reading other higher mathematical literature. ... to be expanded Quantifiers Sometimes we need propositions that involve some description of rough quantity, e.g. "For all odd integers x, x2 is also odd". The word all is a description of quantity. The word "some" is also used to describe quantity. Two special symbols are used to describe the quanties "all" and "some" means "for all" or "for any" means "there are some" or "there exists" Example The proposition: For all even integers x, x2 is also even. can be expressed symbolically as: 1 Example The proposition: There are some odd integers x, such that x2 is even. can be expressed symbolically as: 2 81 This proposition is false. Example Consider the proposition concerning (z = x'y' + xy): For any value of x, there exist a value for y, such that z = 1. can be expressed symbolically as: 3 This proposition is true. Note that the order of the quantifiers is important. While the above statement is true, the statement is false. It asserts that there is one value of y which is the same for all x for which z=1. The first statement only asserts that there is a y for each x, but different values of x may have different values of y. Negation Negation is just a fancy word for the opposite, e.g. The negation of "All named Britney can sing" is "Some named Britney can't sing". What this says is that to disprove that all people named Britney can sing, we only need to find one named Britney who can't sing. To express symbolically: Let p represent a person named Britney Similarly, to disprove we only need to find one odd number that doesn't satisfy the condition. Three is odd, but 3Ã-3 = 9 is also odd, therefore the proposition is FALSE and is TRUE In summary, to obtain the negation of a proposition involving a quantifier, you replace the quantifier by its opposite (e.g. with ) and the quantified proposition (e.g. "x is even") by its negation (e.g. "x is odd"). Example 1 82 is a true statement. Its negation is Axioms and Inference If today's mathematicians were to describe the greatest achievement in mathematics in the 20th century in one word, that word will be abstraction. True to its name, abstraction is a very abstract concept (see Abstraction). In this chapter we shall discuss the essence of some of the number systems we are familiar with. For example, the real numbers and the rational numbers. We look at the most fundamental properties that, in some sense, define those number systems. We begin our discussion by looking at some of the more obscure results we were told to be true • • 0 times any number gives you 0 a negative number multiplied by a negative number gives you a positive number Most people simply accept that they are true (and they are), but the two results above are simple consequences of what we believe to be true in a number system like the real numbers! To understand this we introduce the idea of axiomatic mathematics (mathematics with simple assumptions). An axiom is a statement about a number system that we assume to be true. Each number system has a few axioms, from these axioms we can draw conclusions (inferences). Let's consider the Real numbers, it has axioms Let a, b and c be real numbers For a, b, and c taken from the real numbers A1: a+b is a real number also (closure) A2: There exist 0, such that 0 + a = a for all a (existence of zero - an identity) A3: For every a, there exist b (written -a), such that a + b = 0 (existence of an additive inverse) A4: (a + b) + c = a + (b + c) (associativity of addition) A5: a + b = b + a (commutativity of addition) For a, b, and c taken from the real numbers excluding zero M1: ab (closure) M2: There exist an element, 1, such that 1a = a for all a (existence of one - an identity) 83 M3: For every a there exists a b such that ab = 1 M4: (ab)c = a(bc) (associativity of multiplication) M5: ab = ba (commutativity of multiplication) D1: a(b + c) = ab + ac (distributivity) These are the minimums we assume to be true in this system. These are minimum in the sense that everything else that is true about this number system can be derived from those axioms! Let's consider the following true identity (x + y)z = xz + yz which is not included in the axioms, but we can prove it using the axioms. We proceed: Before we proceed any further, you will have notice that the real numbers are not the only numbers that satifies those axioms! For example the rational numbers also satify all the axioms. This leads to the abstract concept of a field. In simple terms, a field is a number system that satisfies all those axiom. Let's define a field more carefully: A number system, F, is a field if it supports + and Ã- operations such that: For a, b, and c taken from F A1: a + b is in F also (closure) A2: There exist 0, such that 0 + a = a for all a (existence of zero - an identity) A3: For every a, there exist b (written -a), such that a + b = 0 (existence of an additive inverse) A4: (a + b) + c = a + (b + c) (associativity of addition) A5: a + b = b + a (commutativity of addition) For a, b, and c taken from F with the zero removed (sometimes written F*) M1: ab is in F (closure) M2: There exist an element, 1, such that 1a = a for all a (existence of one - the identity) M3: For every a there exists a b such that ab = 1 (inverses) M4: (ab)c = a(bc) (associativity of multiplication) 84 M5: ab = ba (commutativity of multiplication) D1: a(b + c) = ab + ac (distributivity) Now, for M3, we do not let b be zero, since 1/0 has no meaning. However for the M axioms, we have excluded zero anyway. For interested students, the requirements of closure, identity, having inverses and associativity on an operation and a set are known as a group. If F is a group with addition and F* is a group with multiplication, plus the distributivity requirement, F is a field. The above axioms merely state this fact in full. Note that the natural numbers are not a field, as M3 is general not satified, i.e. not every natural number has an inverse that is also a natural number. Please note also that (-a) denotes the additive inverse of a, it doesn't say that (-a) = (-1)(a), although we can prove that they are equivalent. Example 1 Prove using only the axioms that 0 = -0, where -0 is the additive inverse of 0. Solution 1 0 = 0 + (-0) by A3: existence of inverse 0 = (-0) by A2: 0 + a = a Example 2 Let F be a field and a an element of F. Prove using nothing more than the axioms that 0a = 0 for all a. Solution 0 = 0a + (-0a) by A3 existence of inverse 0 = (0 + 0)a + (-0a) by Example 1 0 = (0a + 0a) + (-0a) by distributivity and commutativity of multiplication 0 = 0a + (0a + (-0a)) by associativity of addition 0 = 0a + 0 by A3 0 = 0a by A2. Example 3 Prove that (-a) = (-1)a. 85 Solution 3 (-a) = (-a) + 0 (-a) = (-a) + 0a by Example 2 (-a) = (-a) + (1 + (-1))a (-a) = (-a) + (1a + (-1)a) (-a) = (-a) + (a + (-1)a) (-a) = ((-a) + a) + (-1)a (-a) = 0 + (-1)a (-a) = (-1)a One wonders why we need to prove such obvious things (obvious since primary school). But the idea is not to prove that they are true, but to practise inferencing, how to logically join up arguments to prove a point. That is a vital skill in mathematics. Exercises 1. Describe a field in which 1 = 0 2. Prove using only the axioms if u + v = u + w then v = w (subtracting u from both sides is not accepted as a solution) 3. Prove that if xy = 0 then either x = 0 or y = 0 4. In F-, the operation + is defined to be the difference of two numbers and the Ã- operation is defined to be the ratio of two numbers. E.g. 1 + 2 = -1, 5 + 3 = 2 and 9Ã-3 = 3, 5Ã-2; = 2.5. Is F- a field? 5. Explain why Z6 (modular arithmetic modular 6) is not a field. Problem Set 1. Prove for 2. Prove by induction that 3. Prove by induction 86 where and and 0! = 1 by definition. 4. Prove by induction 5. Prove that if x and y are integers and n an odd integer then is an integer. 6. Prove that (n~m) = n!/((n-m)!m!) is an integer. Where n! = n(n-1)(n-2)...1. E.g 3! = 3Ã-2Ã-1 = 6, and (5~3) = (5!/3!)/2! = 10. Many questions in other chapters require you to prove things. Be sure to try the techniques discussed in this chapter. Feedback What do you think? Too easy or too hard? Too much information or not enough? How can we improve? Please let us know by leaving a comment in the discussion section. Better still, edit it yourself and make it better. 87 Exercises Mathematical proofs At the moment, the main focus is on authoring the main content of each chapter. Therefore this exercise solutions section may be out of date and appear disorganised. If you have a question please leave a comment in the "discussion section" or contact the author or any of the major contributors. Mathematical induction exercises 1. Prove that 12 + 22 + ... + n2 = n(n+1)(2n+1)/6 When n=1, L.H.S. = 12 = 1 R.H.S. = 1*2*3/6 = 6/6 = 1 Therefore L.H.S. = R.H.S. Therefore this is true when n=1. Assume that this is true for some positive integer k, i.e. 12 + 22 + ... + k2 = k(k+1)(2k+1)/6 Therefore this is also true for k+1. Therefore, by the principle of mathematical induction, this holds for all positive integer n. 2. Prove that for n ≥ 1, where xn and yn are integers. 88 When n=1, Therefore x1=1 and y1=1, which are both integers. Therefore this is true when n=1. Assume that this is true for some positive integer k, i.e. where xk and yk are integers. Because xk and yk are both integers, therefore xk + 5yk and xk + yk are integers also. Therefore this is true for k+1 also. Therefore, by the principle of mathematical induction, this holds for all positive integer n. 3. (The solution assume knowledge in binomial expansion and summation notation) Note that Prove that there exists an explicit formula for for all integer m. E.g. It's clear that 11 + 21 + ... = (n+1)n/2. So the proposition is true for m=1. Suppose that 89 has an explicit formula in terms of n for all j < k (**), we aim to prove that also has an explicit formula. Starting from the property given, i.e. Since we know the formula for power sum of any power less then k (**), we can solve the above equation and find out the formula for the k-th power directly. Hence, by the principle of strong mathematical induction, this proposition is true. Additional info for question 3 The method employed in question 3 to find out the general formula for power sum is called the method of difference, as shown by that we consider the sum of all difference of adjacant terms. Aside from the method above, which lead to a recursive solution for finding the general formula, there're also other methods, such as that of using generating function. Refer to the last question in the 90 generating function project page for detail. 91 Problem set Mathematical Proofs Problem Set 1. For all Therefore When a>b and c>d, a+c>b+d ( See also Replace it if you find a better one). Therefore we have: 92 3.Let us call the proposition be P(n) Assume this is true for some n, then Now using the identities of this function:(Note:If anyone find wikibooks ever mentioned this,include a link here!),we have: Since for all n, Therefore P(n) implies P(n+1), and by simple substitution P(0) is true. Therefore by the principal of mathematical induction, P(n) is true for all n. Alternate Notice that solution letting a = b = 1, we get 93 as required. 5. Let be a polynomial with x as the variable, y and n as constants. Therefore by factor theorem(link here please), (x-(-y))=(x+y) is a factor of P(x). Since the other factor, which is also a polynomial, has integer value for all integer x,y and n(I've skipped the part about making sure all coeifficients are of integer value for this moment), it's now obivous that is an integer for all integer value of x,y and n when n is odd. 94 Infinity and infinite processes Introduction As soon as a child first learns about numbers, they become interested in big ones, a million, a billion, a trillion. They even make up their own, a zillion etc. One of the first mathematical questions a child asks is "what is the largest number?" This will often lead to a short explanation that there are infinitely many numbers. But there are many different types of infinity - in fact, there are infinite types of infinity! This chapter will try to explain what some of these types mean and the differences between them. Infinite Sets How big is infinity? Infinity is unlike a normal number because, by definition, it is not finite. Dividing infinity by any positive number (except infinity) gives us infinity. You can also multiply it by anything except zero (or infinity) and it will not get bigger. So let's look more carefully at the different types of infinity. There was once a mathematician called Georg Cantor who created a new branch of mathematics called set theory in the late 19th century. Set theory involves collections of numbers or objects. Here's a set: {1,2,3,4,5} Is it the same size as this one? {6,7,8,9,10} Cantor's notion of sets being "the same size" does not consider whether the numbers are bigger, but whether there are the same amount of objects in it. You can easily see here that they are the same size, because you can simply count the number of members in each set. But with an infinite number of members you cannot, in a finite amount of time, count all the members of one set to see if they are the same number of them as there are in another set. In order to decide if two infinite sets have the same number of members we need to think carefully about what we do when we count. Think of a small child sharing out sweets, between her and her brother. "One for you, and one for me, two for you and two for me" and so on. She knows that they both get the same number of sweets because of the way the sharing out was done. Even if she runs out of numbers (like if she can only count up to ten) she can still distribute the candy with the even process of "another one for you and another one for 95 me". We can use the same idea to compare infinite sets. If we can find a way to pair up one member of set A with one member of set B, and if there are no members of A without a partner in B and vice versa then we can say that set A and set B have the same number of members. Example Let Set N be all counting numbers. N is called the set of natural numbers. 1,2,3,4,5,6, ... and so to infinity. Let Set B be the negative numbers -1,-2,-3, ... and so on to -infinity. Can the members of N and B be paired up? The formal way of saying this is "Can A and B be put into a one to one correspondance"? Obviously the answer is yes. 1 in set N corresponds with -1 in B. Likewise: N B 1 -1 2 -2 3 -3 and so on. So useful is the set of counting numbers that any set that can be put into a one to one correspondence with it is said to be countably infinite. Let's look at some more examples. Is the set of integers countably infinite? Integers are set N, set B and 0. ... -3,-2,-1, 0, 1, 2, 3, ... Historically this set is usually called Z. Note that N the set of natural numbers is a subset of Z. All members of N are in Z, but not all members of Z are in N. What we need to find out is if Z can be put into a one to one correspondence with N. Your first answer, given that N is a subset of Z, may be no but you would be wrong! In set theory, the order of the elements is unimportant. There is no reason why we can't rearrange the elements into any order we please as long as we don't leave any out. Z as presented above doesn't look countable, but if we rewrite it as 0, -1, 1, -2, 2, -3, 3 ..... and so on we can see that it is countable. Z N 0 1 -1 2 1 3 96 -2 4 and so on. Strange indeed! A subset of Z (namely the natural numbers) has the same number of members as Z itself? Infinite sets are not like ordinary finite sets. In fact this is sometimes used as a definition of an infinite set. An infinite set is any set which can be put into a one to one correspondence with at least one of its subsets. Rather than saying "The number of members" of a set, people sometimes use the word cardinality or cardinal value. Z and N are said to have the same cardinality. Exercises 1. Is the number of even numbers the same as the natural numbers? 2. What about the number of square numbers? 3. Is the cardinality of positive even numbers less than 100 equal to the cardinality of natural numbers less than 100? Which set is bigger? How do you know? In what ways do finite sets differ from infinite ones? 4. Using the idea of one to one correspondance prove that infinity + 1 = infinity, what about infinity + A where A is a finite set? What about infinity plus C where C is a countably infinite set? Is the set of rational numbers bigger than N? In this section we will look to see if we can find a set that is bigger than the countable infinity we have looked at so far. To illustrate the idea we can imagine a story. There was once a criminal who went to prison. The prison was not a nice place so the poor criminal went to the prison master and pleaded to be let out. She replied: "Oh all right - I'm thinking of a number, every day you can have a go at guessing it. If you get it correct, you can leave." Now the question is - can the criminal get himself out of jail? (Think about if for a while before you read on) Obviously it depends on the number. If the prison master chooses a natural number, then the criminal guesses 1, to first day, 2,the second day and so on until he reaches the correct number. Likewise for the integers 0 on the first day, -1 on the second day. 1 on the third day and so on. If the number is very large then it may take a long time to get out of prison but get out he will. What the prison master needs to do is choose a set that is not countable in this way. Think of a number line. The integers are widely spaced out. There are plenty of numbers inbetween the integers 0&5 for example. So we need to look at denser sets. The first set that springs to most peoples mind are the fractions. There are an infinite number of fractions between 0 and 1 so surely there are more fractions than integers? Is it possible to count fractions? Let's think about that possibility for a while. If we try to use the approach of counting all the fractions between 0 & 1 then go on to 1 - 2 and so on we will come unstuck becuase we will never finish counting 97 the ones up to 1 ( there are an infinite number of them). But does this mean that they are uncountable ? Think of the situation with the integers. Ordering them ...-2, -1, 0, 1, 2, ... renders them impossible to count, but reordering them 0, -1, 1, -2, 2, ... allows them to be counted. There is in fact a way of ordering fractions to allow them to be counted. Before we go on to it let's revert to the normal mathematical language. Mathematicians use the term rational number to define what we have been calling fractions. A rational number is any number that can be written in the form p/q where p and q are integers. So 3/4 is rational, as is -1/2. The set of all rational numbers is usally called Q. Note that Z is a subset of Q becuase any integer can be divided by 1 to make it into a rational. E.g. the number 3 can be written in the form p/q as 3/1. Now as all the numbers in Q are defined by two numbers p and q it makes sense to write Q out in the form of a table. Note that this table isn't an exact representation of Q. It only has the positive members of Q and has a number of multiple entries.( e.g. 1/1 and 2/2 are the same number) We shall call this set Q'. It is simple enough to see that if Q' is countable then so is Q. So how do we go about counting Q'? If we try counting the first row then the second and so on we will fail because the rows are infinite in length. Likewise if we try to count columns. But look at the diagonals. In one direction they are infinite ( e.g. 1/1, 2/2, 3/3, ...) but in the other direction they are finite. So this set is countable. We count them along the finite diagonals, 1/1, 1/2, 2/1, 1/3, 2/2, 3/1.... Exercises 1. Adapt the method of counting the set Q' to show that thet Q is also countable. How will you include 0 and the negative rationals? How will you solve the problem of multiple entries representing the same number ? 2. Show that (provided that the infinites are both countable) Can we find any sets that are bigger than N? So far we have looked at N, Z, and Q and found them all to be the same size, even though N is a subset of Z which is a subset of Q. You might be beginning to think "Is that it? Are all infinities the same size?" In this section we will look at an set that is bigger than N. A set that cannot be put into a one to one correspondence with N no matter how it is arranged. 98 The set in question is R the real numbers. A real number is any number on the number line that is not in Q. Remember that the set Q contains all the numbers that can be written in the form p/q with p and q rational. Most numbers can never be put in this form. Examples of irrational numbers include Ï€,e, and . The set R is huge! Much bigger than Q. To get a feel for the different sizes of these two infinite sets consider the decimal expansions of a real number and a rational number. Rational numbers always either terminate: • 1/8 = 0.125 or repeat: • 1/9 = 0.1111111...... Imagine measuring an object such as a book. If you use a ruler you might get 10cm. If you take a bit more care to and read the mm you might get 10.2cm. You'd then have to go on to more accurate measing devices such as vernier micrometers and find that you get 10.235cm. Going onto a travelling microscope you may find its 10.235823cm and so on. In general the decimal expansion of any real measurement will be a list of digits that look completely random. Now imagine you measure a book and found it to be 10.101010101010cm. You'd be pretty surprised wouldn't you? But this is exactly the sort of result you would need to get if the book's length were rational. Rational numbers are dense (you find them all over the number line), infinite, yet much much rarer than real numbers. How we can prove that R is bigger than Q It's good to get a feel for the size of infinities as in the previous section. But to be really sure we have to come up with some form of proof. In order to prove that R is bigger than Q we use a classic method. We assume that R is the same size as Q and come up with a contradiction. For the sake of clarity we will restrict our proof to the real numbers between 0 and 1.We will call this set R' Clearly if we can prove that R' is bigger than Q then R must be bigger than Q also. If R' was the same size as Q it would mean that it is countable. This means that we would be able to write out some form of list of all the members of R (This is what countable means, so far we have managed to write out all our infinite sets in the form of an infinitely long list). Let's consider this list. R1 R2 R3 R4 . 99 . . Where R1 is the first number in our list, R2 is the second, and so on. Note that we haven't said what order the list is to be written. For this proof we don't need to say what the order of the list needs to be, only that the mermbers of R are listable (hence countable). Now lets write out the decimal expansion of each of the numbers in the list. 0.r11r12r13r14... 0.r21r22r23r24... 0.r31r32r33r44... 0.r41r42r43r44... . . . Here r11 means the first digit after the decimal point of the first number in the list. So if our first number happened to be 0.36921... r11 would be 3, r12 would be 6 and so on. Remember that this list is meant to be complete. By that we mean that it contains every member of R'. What we are going to do in order to prove that R is not countable is to construct a number that is not already on the list. Since the list is supposed to contain every member of R', this will cause a contradiction and therfore show that R' is unlistable. In order to construct this unlisted number we choose a decimal representation: 0.a1a2a3a4... Where a1 is the first digit after the point etc. We let a1 take any value from 0 - 9 inclusive except the digit r11. So if r11 = 3 then a1 can be 0, 1, 2, 4, 5, 6, 7, 8, or 9. Then we let a2 be any digit except r22 (the second digit of the second number on the list). Then a3 be any digit except r33 and so on. Now if this number, that we have just constructed were on the list somewhere then it would have to be equal to Rsomething. Let's see what Rsomething it might be equal to. It can't be equal to R1 because it has a different first digit (r11 and a1. Nor can it be equal to R2 because it has a different second digit, and so on. In fact it can't be equal to any number on the list becuse it differs by at least one digit from all of them. We have done what we set out to do. We have constructed a number that is in R but is not on the "list of all members of R". This means that R is bigger than any list. It is not listable. It is 100 not countable. It is a bigger infinity than Q. Are there even bigger infinities? There are but they are difficult to describe. The set of all the possible combinations of any number of real numbers is a bigger infinity than R. However trying to imagine such a set is mind boggling. Let's look instead at a set that looks like it should be bigger than R but turns out not to be. Remember R', which we defined earlier on as the set of all numbers on the number line between 0 and 1. Let us now consider the set of all numbers in the plane from [0,0] to [1,1]. At first sight it would seem obvious that there must be more points on the whole plane than there are in a line. But in transfinite mathematics the "obvious" is not always true and proof is the only way to go. Cantor spent three years trying to prove it true but failed. His reason for failure was the best possible. It's false. Each point in this plane is specified by two numbers, the x coordinate and the y coordinate; x and y both belong to R. Lets consider one point in the line. 0.a1a2a3a4.... Can you think of a way of using this one number to specify a point in the plane ? Likewise can you think of a way of combining the two numbers x= 0.x1x2x3x4.... and y= 0.y1y2y3y4.... to specify a point on the line? (think about it before you read on) One way is to do it is to take a1 = x1 a2 = y1 a3 = x2 a4 = y2 . 101 . . This defines a one to one correspondence between the points in the plane and the points in the line. (Actually, for the sharp amongst you, not quite one to one. Can you spot the problem and how to cure it?) Exercises 1. Prove that the number of points in a cube is the same as the number of points on one of its sides. Continuum hypothesis We shall end the section on infinite sets by looking at the Continuum hypothesis. This hypothesis states that there are no infinities between the natural numbers and the real numbers. Cantor came up with a number system for transfinite numbers. He called the smallest infinity with the next biggest one and so on. It is easy to prove that the cardinality of N is (Write any smaller infinity out as a list. Either the list terminates, in which case it's finite, or it ? goes on forever, in which case it's the same size as N) but is the cardinality of the reals = To put it another way, the hypotheses states that: There are no infinite sets larger than the set of natural numbers but smaller than the set of real numbers. The hypothesis is interesting because it has been proved that "It is not possible to prove the hypothesis true or false, using the normal axioms of set theory" Further reading If you want to learn more about set theory or infinite sets try one of the many interesting pages on our sister project en:wikipedia. • • • • ordinal numbers Aleph numbers Set theory Hilbert's Hotel Limits Infinity got rid of The theory of infinite sets seems weird to us in the 21st century, but in Cantor's day it was downright unpalatable for most mathematicians. In those days the idea of infinity was too troublesome, they tried to avoid it wherever possible. 102 Unfortunately the mathematical topic called analysis was found to be highly useful in mathematics, physics, engineering. It was far too useful a field to simply drop yet analysis relies on infinity or at least infinite processes. To get around this problem the idea of a limit was invented. Consider the series This series is called the harmonic series. Note that the terms of the series get smaller and smaller as you go further and further along the series. What happens if we let n become infinite? The term would become But this doesn't make sense. (Mathematicians consider it sloppy to divide by infinity. Infinity is not a normal number, you can't divide by it). A better way to think about it (The way you probably already do think about it, if you've ever considered the matter) is to take this approach: Infinity is very big, bigger than any number you care to think about. So let's let n become bigger and bigger and see if 1/n approaches some fixed number. In this case as n gets bigger and bigger 1/n gets smaller and smaller. So it is reasonable to say that the limit is 0. In mathematics we write this as and it reads: the limit of 1/n as n approaches infinity is zero Note that we are not dividing 1 by infinity and getting the answer 0. We are letting the number n get bigger and bigger and so the reciprocal gets closer and closer to zero. Those 18th Century mathematicians loved this idea because it got rid of the pesky idea of dividing by infinity. At all times n remains finite. Of course, no matter how huge n is, 1/n will not be exactly equal to zero, there is always a small difference. This difference (or error) is usually denoted by Îµ (epsilon). info -- infinitely small When we talk about infinity, we think of it as something big. But there is also the infinitely small, denoted by Îµ (epsilon). This animal is closer to zero than any other number. Mathematicians also use the character Îµ to represent anything small. For example, the famous Hungarian mathematician Paul Erdos used to refer to small children as epsilons. 103 Examples Lets look at the function What is the limit as x approaches infinity ? This is where the idea of limits really come into its own. Just replacing x with infinity gives us very little: But by using limits we can solve it For our second example consider this limit as x approaches infinity of x3 - x2 Again lets look at the wrong way to do it. Substituting answer zero. Now lets look at doing it the correct way, using limits into the expression gives . Note that you cannot say that these two infinities just cancel out to give the The last expression is two functions multiplied together. Both of these functions approach infinity as x approaches infinity, so the product is infinity also. This means that the limit does not exist, i.e. there is no finite number that the function approaches as x gets bigger and bigger. One more just to get you really familer with how it works. Calculate: To make things very clear we shall rewrite it as Now to calculate this limit we need to look at the properties of sin(x). Sin(x)is a function that you should already be familiar with (or you soon will be) its value oscillates between 1 and -1 104 depending on x. This means that the absolute value of sin(x) (the value ignoring the plus or minus sign) is always less than or equal to 1: So we have 1/x which we already know goes to zero as x goes to infinity multiplied by sin(x) which always remains finite no matter how big x gets. This gives us Exercises Evaluate the following limits; 1. 2. 3. 4. Infinite series Consider the infinite sum 1/1 + 1/2 + 1/4 + 1/8 + 1/16 + .... Do you think that this sum will equal infinity once all the terms have been added ? Let's sum the first few terms. Can you guess what is ? Here is another way of looking at it. Imagine a point on a number line moving along as the sum progresses. In the first term the point jumps to the position 1. This is half way from 0 to 2. In the second stage the point jumps to position 1.5 - half way from 1 to 2. At each stage in the process (shown in a different colour on the diagram) the distance to 2 is halved. The point can 105 get as close to the point 2 as you like. You just need to do the appropriate number of jumps, but the point will never actually reach 2 in a finite number of steps. We say that in the limit as n approaches infinity, Sn approaches 2. Zeno's Paradox The ancient Greeks had a big problem with summing infinite series. A famous paradox from the philosopher Zeno is as follows: In the paradox of Achilles and the tortoise, we imagine the Greek hero Achilles in a footrace with the plodding reptile. Because he is so fast a runner, Achilles graciously allows the tortoise a head start of a hundred feet. If we suppose that each racer starts running at some constant speed (one very fast and one very slow), then after some finite time, Achilles will have run a hundred feet, bringing him to the tortoise's starting point. During this time, the tortoise has "run" a (much shorter) distance, say one foot. It will then take Achilles some further period of time to run that distance, during which the tortoise will advance farther; and then another period of time to reach this third point, while the tortoise moves ahead. Thus, whenever Achilles reaches somewhere the tortoise has been, he still has farther to go. Therefore, Zeno says, swift Achilles can never overtake the tortoise. Feedback What do you think? Too easy or too hard? Too much information or not enough? How can we improve? Please let us know by leaving a comment in the discussion section. Better still, edit it yourself and make it better. 106 Exercises Infinity and infinite processes At the moment, the main focus is on authoring the main content of each chapter. Therefore this exercise solutions section may be out of date and appear disorganised. If you have a question please leave a comment in the "discussion section" or contact the author or any of the major contributors. These solutions were not written by the author of the rest of the book. They are simply the answers I thought were correct while doing the exercises. I hope these answers are usefull for someone and that people will correct my work if I made some mistakes How big is infinity? exercises 1. The number of even numbers is the same as the number of natural numbers because both are countably infinite. You can clearly see the one to one correspondence. (E means even numbers and is not an official set like N) E N 2 1 4 2 6 3 8 4 2. The number of square numbers is also equal to the number of natural numbers. They are both countably infinite and can be put in one to one correspondence. (S means square numbers and is not an official set like N) S N 1 1 4 2 9 3 16 4 3. The cardinality of even numbers less than 100 is not equal to the cardinality of natural numbers less than 100. You can simply write out both of them and count the numbers. Then you will see that cardinality of even numbers less than 100 is 49 and the cardinality of natural numbers less than 100 is 99. Thus the set of natural numbers less than 100 is bigger than the set 107 of even numbers less than 100. The big difference between infinite and finite sets thus is that a finite set can not be put into one to one correspondence with any of it's subsets. While an infinite set can be put into one to one correspondence with at least one of it's subsets. 4. Each part of the sum is answered below infinity + 1 = infinity You can prove this by taking a set with a cardinality of 1, for example a set consisting only of the number 0. You simply add this set in front of the countably infinite set to put the infinite set and the inifinite+1 set into one to one correspondence. N N+1 1 0 2 1 3 2 4 3 infinity + A = infinity (where A is a finite set) You simply add the finite set in front of the infinite set like above, only the finite set doesn't need to have a cardinality of one anymore. infinity + C = infinity (where C is a countably infinite set) You take one item of each set (infinity or C) in turns, this will make the new list also countably infinite. Is the set of rational numbers bigger than N? exercises 1. To change the matrix from Q' to Q the first step you need to take is to remove the multiple entries for the same number. You can do this by leaving an empty space in the table when gcd(topnr,bottomnr) ≠ 1 because when the gcd isn't 1 the fraction can be simplified by dividing the top and bottom number by the gcd. This will leave you with the following table. Now we only need to add zero to the matrix and we're finished. So we add a vertical row for zero and only write the topmost element in it (0/1) (taking gcd doesn't work here because 108 gcd(0,a)=a) This leaves us with the following table where we have to count all fractions in the diagonal rows to see that Q is countably infinite. 2. To show that you have to make a table where you put one infinity in the horizontal row and one infinity in the vertical row. Now you can start counting the number of place in the table diagonally just like Q' was counted. This works because a table of size AxB contains A*B places. Are there even bigger infinities? exercises 1. You have to use a method to map the coordinates in a plain onto a point on the line and the other way around, like the one described in the text. This method shows you that for every number on the line there is a place on the plain and for every place on the plain there is a place on the line. Thus the number of points on the line and the plain are the same. Limits Infinity got rid of exercises 1. 2. 3. 4. Problem set HSE PS Infinity and infinite processes 109 Counting and Generating functions Before we begin: This chapter assumes knowledge of 11. 12. 13. Ordered selection (permutation) and unordered selection (combination) covered in Basic counting, Method of Partial Fractions and, Competence in manipulating Summation Signs Some Counting Problems ..more to come Generating functions ..some motivation to be written Generating functions, otherwise known as Formal Power Series, are useful for solving problems like: x1 + x2 + 2x3 = m where ; n = 1, 2, 3 how many unique solutions are there if m = 55? Before we tackle that problem, let's consider the infinite polynomial: S = 1 + x + x2 + x3 + ... + xn + xn + 1... We want to obtain a closed form of this infinite polynomial. The closed form is simply a way of expressing the polynomial so that it involves only a finite number of operations. So the closed form of 1 + x + x2 + x3 + ... 110 is We can equate them (actually, we can't. Refer to info). info - Infinite sums The two expressions are not equal. It's just that for certain values of x (-1 < x < 1), we can approximate the right hand side as closely as possible by adding up a large number of terms on the left hand side. For example, suppose x = 1/2, RHS = 2; we approximate the LHS using only 5 terms we get LHS equals 1 + 1/2 + 1/4 + 1/8 + 1/16 = 1.9375, which is close to 2, as you can imagine by adding more and more terms, we will get closer and closer to 2. For a more detailed discussion of the above, head to Infinity and infinite processes. Anyway we really only care about its nice algebraic properties not its numerical value. From now on we will omit the condition for equality to be true when writing out generating functions. Consider a more general case: S = A + ABx + AB2x2 + AB3x3 + ... where A and B are constants. We can derive the closed-form as follows: The following identity as derived above is worth investing time and effort memorising. Exercises 1. Find the closed-form: 111 (a)1 - z + z2 - z3 + z4 - z5 + ... (b)1 + 2z + 4z2 + 8z3 + 16z4 + 32z5 + ... (c)z + z2 + z3 + z4 + z5 + ... (d)3 - 4z + 4z2 - 4z3 + 4z4 - 4z5 + ... (e)1 - z2 + z4 - z6 + z8 - z10 + ... 2. Given the closed-form, find a function f(n) for the coefficients of xn: (a) (Hint: note the plus sign in the denominator) (Hint: obtain the generating function for 1/(1 - z^2) first, then multiply by the (b) appropriate expression) (c) (Hint: break into the sum of two distinct close forms) Method of Substitution We are given that: 1 + z + z2 + ... = 1/(1 - z) and we can obtain many other generating functions by substitution. For example: letting z = x2 we have: 1 + x2 + x4 + ... = 1/(1 - x2) Similarly A + ABx + A(Bx)2 + ... = A/(1 - Bx) is obtained by letting z = Bx then multiplying the whole expression by A. Exercises 1. What are the coefficients of the powers of x: 1/(1 - 2x3) 2. What are the coefficients of the powers of x (Hint: take out a factor of 1/2): 1/(2 - x) 112 Linear Recurrence Relations The Fibonacci series 1, 1, 2, 3, 5, 8, 13, 21, 34, 55... where each and every number, except the first two, is the sum of the two preceding numbers. We say the numbers are related if the value a number takes depends on the values that come before it in the sequence. The Fibonacci sequence is an example of a recurrence relation, it is expressed as: where xn is the (n+ 1)th number in the sequence. Note that the first number in the sequence is denoted x0. Given this recurrence relation, the question we want to ask is "can we find a formula for the (n+1)th number in the sequence?". The answer is yes, but before we come to that, let's look at some examples. Example 1 The expressions define a recurrence relation. The sequence is: 1, 1, 5, 13, 41, 121, 365... Find a formula for the (n+1)th number in the sequence. Solution Let G(z) be generating function of the sequence, meaning the coefficient of each power (in ascending order) is the corresponding number in the sequence. So the generating functions looks like this G(z) = 1 + z + 5z2 + 13z3 + 41z4 + 121z5 + ... Now, by a series of algebraic manipulations, we can find the closed form of the generating function and from that the formula for each coefficient 113 by definition xn - 2xn - 1 - 3xn - 2 = 0 by the method of partial fractions we get: each part of the sum is in a recognisable closed-form. We can conclude that: the reader can easily check the accuracy of the formula. Example 2 Find a non-recurrent formula for xn. Solution Let G(z) be the generating function of the sequence described above. G(z) = x0 + x1z + x2z2 + ... 114 Therefore xn = 1, for all n. Example 3 A linear recurrence relation is defined by: Find the general formula for xn. Solution Let G(z) be the generating function of the recurrence relation. G(z)(1 - z - 6z2) = x0 + (x1 - x0)z + (x2 - x1 - 6x0)z2 + ... G(z)(1 - z - 6z2) = 1 + z2 + z3 + z4 + ... G(z)(1 - z - 6z2) = 1 + z2(1 + z + z2 + ...) Therefore Exercises 1. Derive the formula for the (n+1)th numbers in the sequence defined by the linear recurrence relations: 115 2. Derive the formula for the (n+1)th numbers in the sequence defined by the linear recurrence relations: 3. (Optional) Derive the formula for the (n+1)th Fibonacci numbers. Further Counting Consider the equation a + b = n; a, b ≥ 0 are integers For a fixed positive integer n, how many solutions are there? We can count the number of solutions: 0+n=n 1 + (n - 1) = n 2 + (n - 2) = n ... n+0=n As you can see there are (n + 1) solutions. Another way to solve the problem is to consider the generating function G(z) = 1 + z + z2 + ... + zn Let H(z) = G(z)G(z), i.e. H(z) = (1 + z + z2 + ... + zn)2 I claim that the coefficient of zn in H(z) is the number of solutions to a + b = n, a, b > 0. The reason why lies in the fact that when multiplying like terms, indices add. Consider A(z) = 1 + z + z2 + z3 + ... Let 116 B(z) = A2(z) it follows B(z) = (1 + z + z2 + z3 + ...) + z(1 + z + z2 + z3 + ...) + z2(1 + z + z2 + z3 + ...) + z3(1 + z + z2 + z3) + ... B(z) = 1 + 2z + 3z2 + ... Now the coefficient of zn (for n ≥ 0) is clearly the number of solutions to a + b = n (a, b > 0). We are ready now to derive a very important result: let tk be the number solutions to a + b = n (a, b > 0). Then the generating function for the sequence tk is T(z) = (1 + z + z2 + ... + zn + ...)(1 + z + z2 + ... + zn + ...) i.e. Counting Solutions to a1 + a2 + ... + am = n Consider the number of solutions to the following equation: a1 + a2 + ... + am = n where ai ≥ 0; i = 1, 2, ... m. By applying the method discussed previously. If tk is the number of solutions to the above equation when n = k. The generating function for tk is but what is tk? Unless you have learnt calculus, it's hard to derive a formula just by looking the equation of T(z). Without assuming knowledge of calculus, we consider the following counting problem. "You have three sisters, and you have n (n ≥ 3) lollies. You decide to give each of your sisters at least one lolly. In how many ways can this be done?" One way to solve the problem is to put all the lollies on the table in a straightline. Since there are n lollies there are (n - 1) gaps between them (just as you have 5 fingers on each hand and 4 gaps between them). Now get 2 dividers, from the (n - 1) gaps available, choose 2 and put a divider in each of the gaps you have chosen! There you have it, you have divided the n lollies 117 into three parts, one for each sister. There are ways to do it! If you have 4 sisters, then there are it. ways to do it. If you have m sisters there are ways to do Now consider: "You have three sisters, and you have n lollies. You decide to give each of your sisters some lollies (with no restriction as to how much you give to each sister). In how many ways can this be done?" Please note that you are just solving: a1 + a2 + a3 = n where ai ≥ 0; i = 1, 2, 3. You can solve the problem by putting n + 3 lollies on the table in a straightline. Get two dividers and choose 2 gaps from the n + 2 gaps available. Now that you have divided n + 3 lollies into 3 parts, with each part having 1 or more lollies. Now take back 1 lollies from each part, and you have solved the problem! So the number of solutions is generally, if you have m sisters and n lollies the number of ways to share the lollies is . More . Now to the important result, as discussed above the number of solutions to a1 + a2 + ... + am = n where ai ≥ 0; i = 1, 2, 3 ... m is i.e. Example 1 The closed form of a generating function T(z) is 118 and tk in the coefficient of zk is T(z). Find an explicit formula for tk. Solution Therefore tk = k Example 2 Find the number of solutions to: a+b+c+d=n for all positive integers n (including zero) with the restriction a, b, c ,d ≥ 0. Solution By the formula so the number of solutions is More Counting We turn to a sligthly harder problem of the same kind. Suppose we are to count the number of solutions to: 2a + 3b + c = n for some integer , with a, b, also c greater than or equal zero. We can write down the closed form straight away, we note the coefficient of xn of: 119 is the required solution. This is due to, again, the fact that when multiplying powers, indices add. To obtain the number of solutions, we break the expression into recognisable closed-forms by method of partial fraction. Example 1 Let sk be the number of solutions to the following equation: 2a + 2b = n; a, b ≥ 0 Find the generating function for sk, then find an explicit formula for sn in terms of n. Solution Let T(z) be the generating functions of tk T(z) = (1 + z2 + z4 + ... + z2n + ...)2 It's not hard to see that Example 2 Let tk be the number of solutions to the following equation: a + 2b = n; a, b ≥ 0 Find the generating function for tk, then find an explicit formula for tn in terms of n. Solution Let T(z) be the generating functions of tk T(z) = (1 + z + z2 + ... + zn + ...)(1 + z2 + z4 + ... + z2n + ...) 120 A = -1/4, B = 3/4, C = 1/4 Exercises 1. Let be the generating functions for tk (k = 0, 1, 2 ...). Find an explicit formula for tk in terms of k. 2. How many solutions are there the following equations if m is a given constant a + b + 2c = m where a, b and c ≥ 0 Problem Set 1. A new Company has borrowed $250,000 initial capital. The monthly interest is 3%. The company plans to repay $x before the end of each month. Interest is added to the debt on the last day of the month (compounded monthly). Let Dn be the remaining debt after n months. a) Define Dn recursively. b) Find the minimum values of x. c) Find out the general formula for Dn. d) Hence, determine how many months are need to repay the debt if x = 12,000. 2. A partion of n is a sequence of positive integers (λ1,λ1,..,λr) such that λ1 ≥ λ2 ≥ .. ≥ λr and 121 λ1 + λ2 + .. + λr = n. For example, let n = 5, then (5), (4,1), (3,2), (3,1,1), (2,2,1), (2,1,1,1), (1,1,1,1,1) are all the partions of 5. So we say the number of partions of 5 is 7. Derive a formula for the number of partions of a general n. 122 3. A binary tree is a tree where each node can have up to two child nodes. The figure below is an example of a binary tree. a) Let cn be the number of unique arrangements of a binary tree with totally n nodes. Let C(z) be a generating function of cn. (i) Define C(z) using recursion. (ii) Hence find the closed form of C(z). b) Let be a power series. (i) By considering the n-th derivative of P(x), find a formula for pn. (ii) Using results from a) and b)(i) , or otherwise, derive a formula for cn. Hint: Instead of doing recursion of finding the change in cn when adding nodes at the buttom, try to think in the opposite way, and direction.(And no, not deleting nodes) 123 Project - Exponential generating function This project assumes knowledge of differentiation. (Optional)0. (a) (i) Differentiate log x by first principle. (ii)*** Show that the remaining limit in last part that can't be evaluated indeed converges. Hence finish the differentiation by assigning this number as a constant. (b) Hence differentiate ax. 1. Consider E(x) = ex (a) Find out the n-th derivative of E(x). (b) By considering the value of the n-th derivative of E(x) at x = 0, express E(x) in power series/infinite polynomial form. (Optional)2. (a) Find out the condition for the geometric progression(that is the ordinary generating function introduced at the begining of this chapter) to converges. (Hint: Find out the partial sum) (b) Hence show that E(x) in the last question converges for all real values of x. (Hint: For any fixed x, the numerator of the general term is exponential, while the denominator of the general term is factorial. Then what?) 3. The function E(x) is the most fundamental and important exponential generating function, it is similar to the ordinary generating function, but with some difference, most obviously having a fractorial fraction attached to each term. (a) Similar to ordinary generating function, each term of the polynomial expansion of E(x) can have number attached to it as coefficient. Now consider Find A'(x) and compare it with A(x). What do you discover? (b) Substitute nz, where n is a real number and z is a free variable, into E(x), i.e. E(nz). What have you found? 4. Apart from A(x) defined in question 2, let 124 (a) What is A(x) multiplied by B(x)? Compare this with ordinary generating function, what is the difference? (b) What if we blindly multiply A(x) with x(or xn in general)? Will it shift coefficient like what happened in ordinary generating function? Notes: Question with *** are difficult questions, although you're not expected to be able to answer those, feel free to try your best. Feedback What do you think? Too easy or too hard? Too much information or not enough? How can we improve? Please let us know by leaving a comment in the discussion section. Better still, edit it yourself and make it better. 125 Exercises Counting and Generating functions At the moment, the main focus is on authoring the main content of each chapter. Therefore this exercise solutions section may be out of date and appear disorganised. If you have a question please leave a comment in the "discussion section" or contact the author or any of the major contributors. These solutions were not written by the author of the rest of the book. They are simply the answers I thought were correct while doing the exercises. I hope these answers are useful for someone and that people will correct my work if I made some mistakes Generating functions exercises 1. (a)S = 1 - z + z2 - z3 + z4 - z5 + ... zS = z - z2 + z3 - z4 + z5 - ... (1 + z)S = 1 (b)S = 1 + 2z + 4z2 + 8z3 + 16z4 + 32z5 + ... 2zS = 2z + 4z2 + 8z3 + 16z4 + 32z5 + ... (1 - 2z)S = 1 (c)S = z + z2 + z3 + z4 + z5 + ... zS = z2 + z3 + z4 + z5 + ... (1 - z)S = z (d)S = 3 - 4z + 4z2 - 4z3 + 4z4 - 4z5 + ... z(S + 1) = 4z - 4z2 + 4z3 - 4z4 + 4z5 - ... 126 S + z(S + 1) = 3 S + zS + z = 3 (1 + z)S = 3 - z 2. (a) S = 1 - x + x2 - x3 + x4 - x5 + ... f(n) = ( - 1)n (b) (1 - z2)S = z3 S = z3 + z5 + z7 + z9 + ... f(n) = 0;for n is odd 2c only contains the exercise and not the answer for the moment (c) Linear Recurrence Relations exercises This section only contains the incomplete answers because I couldn't figure out where to go from here. 1. 127 Let G(z) be the generating function of the sequence described above. G(z) = x0 + x1z + x2z2 + ... (1 - 2z)G(z) = x0 + (x1 - 2x0)z + (x2 - 2x1)z2 + ... (1 - 2z)G(z) = 1 - z - z2 - z3 - z4 - ... (1 - 2z)G(z) = 1 - z(1 + z + z2 + ...) xn = 1 2. Let G(z) be the generating function of the sequence described above. G(z) = x0 + x1z + x2z2 + ... (3 + 4z - z2)G(z) = 3x0 + (3x1 + 4x0)z + (3x2 + 4x1 - x0)z2 + (3x3 + 4x2 - x1)z3 + ... (3 + 4z - z2)G(z) = 3x0 + (3x1 + 4x0)z (3 + 4z - z2)G(z) = 3 + 7z 3. Let G(z) be the generating function of the sequence described above. G(z) = x0 + x1z + x2z2 + ... (1 - z - z2)G(z) = x0 + (x1 - x0)z + (x2 - x1 - x0)z2 + (x3 - x2 - x1)z2 + ... (1 - z - z2)G(z) = 1 128 We want to factorize f(z) = z2 + z - 1 into (z - α)(z - β) , by the converse of factor theorem, if (z p) is a factor of f(z), f(p)=0. Hence α and β are the roots of the quadratic equation z2 + z - 1 = 0 Using the quadratic formula to find the roots: In fact, these two numbers are the faomus golden ratio and to make things simple, we use the greek symbols for golden ratio from now on. Note: is denoted φ and is denoted Φ By the method of partial fraction: 129 Further Counting exercises 1. We know that therefore Thus Tk = ( - 1)k(k + 1) 2. a + b + c = m Thus *Differentiate from first principle* exercises 1. 130 131 Discrete Probability Introduction Probability theory is one of the most widely applicable mathematical theories. It deals with uncertainty and teaches you how to manage it. It is simply one of the most useful theories you will ever learn. Please do not misunderstand. We are not learning to predict things, rather we learn to utilise predicted chances and make them useful. Therefore, we don't care, what is the probability it will rain tomorrow?, but given the probability is 60% we can make deductions, the easiest of which is the probability it will not rain tomorrow is 40%. As suggested above, a probability is a percentage and it's between 0% and 100% (inclusive). Mathematicians like to express a probability as a proportion i.e. as a number between 0 and 1. info - Why discrete? Probability comes in two flavours, discrete and continuous. The continuous case is considered to be far more difficult to understand, and much less intuitive, than discrete probability and it requires knowledge of calculus. But we will touch on a little bit of the continuous case later on in the chapter. Event and Probability Roughly, an event is something we can assign a probability to. For example the probability it will rain tomorrow is 0.6, in here the event is it will rain tomorrow the assigned probability is 0.6. We can write P(it will rain tomorrow) = 0.6 as mathematicians like to do we can use abstract letters to represent events. In this case we choose A to represent the event it will rain tomorrow, so the above expression can be written as P(A) = 0.6 Another example a fair die will turn up 1, 2, 3, 4, 5 or 6 equally probably each time it is tossed. Let B be the event that it turns up 1 in the next toss, we write P(B) = 1/6 Misconception Please note that the probability 1/6 does not mean that it will turns up 1 in at most six tries. Its precise meaning will be discussed later on in the chapter. Roughly, it just means that on the long run (i.e. the die being tossed a large number of times), the proportion of 1's will be 132 very close to 1/6. Impossible and Certain events Two types of events are special. One type are the impossible events (e.g., the sum of digits of a two-digit number is greater than 18); the other type are certain to happen (e.g., a roll of a die will turn up 1, 2, 3, 4, 5 or 6). The probability of an impossible event is 0, while that of a certain event is 1. We write P(Impossible event) = 0 P(Certain event) = 1 The above reinforces a very important principle concerning probability. Namely, the range of probability is between 0 and 1. You can never have a probability of 2.5! So remember the following for all events E. Complement of an event A most useful concept is the complement of an event. We use : to represent the event that the die will NOT turn up 1 in the next toss. Generally, putting a bar over a variable (that represents an event) means the opposite of that event. In the above case of a die: it means the die will turn up 2, 3, 4, 5 or 6 in the next toss has probability 5/6. Please note that for any event E. Combining independent probabilities It is interesting how independent probabilities can be combined to yield probabilities for more complex events. I stress the word independent here, because the following demonstrations will not work without that requirement. The exact meaning of the word will be discussed a little later on in the chapter, and we will show why independence is important in Exercise 10 of this section. Adding probabilities Probabilities are added together whenever an event can occur in multiple "ways." As this is a rather loose concept, the following example may be helpful. Consider rolling a single die; if we want to calculate the probability for, say, rolling an odd number, we must add up the 133 probabilities for all the "ways" in which this can happen -- rolling a 1, 3, or 5. Consequently, we come to the following calculation: P(rolling an odd number) = P(rolling a 1) + P(rolling a 3) + P(rolling a 5) = 1/6 + 1/6 + 1/6 = 3/6 = 1/2 = 50% Note that the addition of probabilities is often associated with the use of the word "or" -whenever we say that some event E is equivalent to any of the events X, Y, or Z occurring, we use addition to combine their probabilities. A general rule of thumb is that the probability of an event and the probability of its complement must add up to 1. This makes sense, since we intuitively believe that events, when welldefined, must either happen or not happen. Multiplying probabilities Probabilities are multiplied together whenever an event occurs in multiple "stages" or "steps." For example, consider rolling a single die twice; the probability of rolling a 6 both times is calculated by multiplying the probabilities for the individual steps involved. Intuitively, the first step is simply the first roll, and the second step is the second roll. Therefore, the final probability for rolling a 6 twice is as follows: P(rolling a 6 twice) = P(rolling a 6 the first time) 1/36 2.8% P(rolling a 6 the second time) = = Similarly, note that the multiplication of probabilities is often associated with the use of the word "and" -- whenever we say that some event E is equivalent to all of the events X, Y, and Z occurring, we use multiplication to combine their probabilities. Also, it is important to recognize that the product of multiple probabilities must be less than or equal to each of the individual probabilities, since probabilities are restricted to the range 0 through 1. This agrees with our intuitive notion that relatively complex events are usually less likely to occur. Combining addition and multiplication It is often necessary to use both of these operations simultaneously. Once again, consider one die being rolled twice in succession. In contrast with the previous case, we will now consider the event of rolling two numbers that add up to 3. In this case, there are clearly two steps involved, and therefore multiplication will be used, but there are also multiple ways in which the event under consideration can occur, meaning addition must be involved as well. The die could turn up 1 on the first roll and 2 on the second roll, or 2 on the first and 1 on the second. This leads to the following calculation: P(rolling a sum of 3) = P(1 on 1st roll) P(2 on 2nd roll) + P(2 on 1st roll) P(1 on 2nd roll) 134 = + = 1/18 5.5% This is only a simple example, and the addition and multiplication of probabilities can be used to calculate much more complex probabilities. Exercises Let A represent the number that turns up in a (fair) die roll, let C represent the number that turns up in a separate (fair) die roll, and let B represent a card randomly picked out of a deck: 1. A die is rolled. What is the probability of rolling a 3 i.e. calculate P(A = 3)? 2. A die is rolled. What is the probability of rolling a 2, 3, or 5 i.e. calculate P(A = 2) + P(A = 3) + P(A = 5)? 3. What is the probability of choosing a card of the suit Diamonds? 4. A die is rolled and a card is randomly picked from a deck of cards. What is the probability of rolling a 4 and picking the Ace of spades, i.e. calculate P(A = 4)×P(B = Ace of spades). 5. Two dice are rolled. What is the probability of getting a 1 followed by a 3? 6. Two dice are rolled. What is the probability of getting a 1 and a 3, regardless of order? 7. Calculate the probability of rolling two numbers that add up to 7. 8. (Optional) Show the probability of C is equal to A is 1/6. 9. What is the probability that C is greater than A? 10. Gareth was told that in his class 50% of the pupils play football, 30% play video games and 30% study mathematics. So if he was to choose a student from the class randomly, he calculated the probability that the student plays football, video games or studies mathematics is 50% + 30% + 30% = 1/2 + 3/10 + 3/10 = 11/10. But all probabilities should be between 0 and 1. What mistake did Gareth make? Solutions 1. P(A = 3) = 1/6 2. P(A = 2) + P(A = 3) + P(A = 5) = 1/6 + 1/6 + 1/6 = 1/2 3. P(B = Ace of Diamonds) + ... + P(B = King of Diamonds) = 13 × 1/52 = 1/4 4. P(A = 4) × P(B = Ace of Spades) = 1/6 × 1/52 = 1/312 5. P(A = 1) × P(A = 3) = 1/36 6. P(A = 1) × P(A = 3) + P(A = 3) × P(A = 1) = 1/36 + 1/36 = 1/18 135 7. Here are the possible combinations: 1 + 6 = 2 + 5 = 3 + 4 = 7. Probability of getting each of the combinations are 1/18 as in Q6. There are 3 such combinations, so the probability is 3 × 1/18 = 1/6. 9. Since both dice are fair, C > A is just as likely as C < A. So P(C > A) = P(C < A) and P(C > A) + P(C < A) + P(A = C) = 1 But P(A = C) = 1/6 so P(C > A) = 5/12. 10. For example, some of those 50% who play football may also study mathematics. So we can not simply add them. Random Variables A random experiment, such as throwing a die or tossing a coin, is a process that produces some uncertain outcome. We also require that a random experiments can be repeated easily. In this section we shall start using a capital letter to represent the outcome of a random experiment. For example, let D be the outcome of a die roll, D could take the value 1, 2, 3, 4, 5 or 6, but it is uncertain. We say D is a random variable. Suppose now I throw a die, and it turns up 5, we say the observed value of D is 5. A random variable is simply the outcome of a certain random experiment. It is usually denoted by a CAPITAL letter, but its observed value is not. For example let D1,D2,...,Dn denote the outcome of n die throws, then we usually use d1,d2,...,dn to denoted the observed values of each of Di's. From here on, random variable may be abbreviated as simply rv (a common abbreviation in other probability literatures). The Bernoulli This section is optional and it assumes knowledge of binomial expansion. A Bernoulli experiment is basically a "coin-toss". If we toss a coin, we will expect to get a head 136 or a tail equally probably. A Bernoulli experiment is slightly more versatile than that, in that the two possible outcomes need not have the same probability. In a Bernoulli experiment you will either get a success, denoted by 1, with proability p (where p is a number between 0 and 1) or a failure, denoted by 0, with probaility 1 - p. If the random variable B is the outcome of a Bernoulli experiment, and the probability of getting a 1 is p, we say B comes from a Bernoulli distribution with success probability p and we write: For example, if then P(C = 1) = 0.65 and P(C = 0) = 1 - 0.65 = 0.35 Binomial Distribution Suppose we want to repeat the Bernoulli experiment n times, then we get a binomial distribution. For example: for i = 1, 2, ... , n. That is, there are n variables C1, C2, ... , Cn and they all come from the same Bernoulli distribution. We consider: B = C1 + C2 + ... + Cn , then B is simply the rv that counts the number of successes in n trials (experiments). Such a variables is called a binomial variable, and we write Example 1 Aditya, Gareth, and John are equally able. Their probability of scoring 100 in an exam follows 137 a Bernoulli distribution with success probability 0.9. What is the probability of i) One of them getting 100? ii) Two of them getting 100? iii) All 3 getting 100? iv) None getting 100? Solution We are dealing with a binomial variable, which we will call B. And i) We want to calculate P(B = 1) The probability of any of them getting 100 (success) and the other two getting below 100 (failure) is but there are 3 possible candidates for getting 100 so ii) We want to calculate P(B = 2) The probability is but there are combinations of candidates for getting 100, so iii) To calculate iv) The probability of "None getting 100" is getting 0 success, so 138 The above example strongly hints at the fact the binomial distribution is connected with the binomial expansion. The following result regarding the binomial distribution is provided without proof, the reader is encouraged to check its correctness. 139 If then This is the kth term of the binomial expansion of (p + q)n, where q = 1 - p. Exercises ... Distribution ... Events In the previous sections, we have slightly abused the use of the word event. An event should be thought of as a collection of random outcomes of a certain rv. Let us introduce some notations first. Let A and B be two events, we define to be the event of A and B. We also define to be the event of A or B. As demonstrated in exercise 10 above, in general. Let's see some examples. Let A be the event of getting a number less than or equal to 4 when tossing a die, and let B be the event of getting an odd number. Now P(A) = 2/3 and P(B) = 1/2 but the probability of A or B does not equal to the sum of the probabilities, as below 140 as 7/6 is greater than 1. It is not difficult to see that the event of throwing a 1 or 3 is included in both A and B. So if we simply add P(A) and P(B), some events' probabilities are being added twice! The Venn diagram below should clarify the situation a little more, think of the blue square as the probability of B and the yellow square as the probability of A. These two probabilities overlap, and where they do is the probability of A and B. So the probability of A or B should be: The above formula is called the Simple Inclusion Exclusion Formula. If for events A and B, we have we say A and B are disjoint. The word means to separate. If two events are disjoint we have the following Venn diagram representing them: 141 info -- Venn Diagram Traditionally, Venn Diagrams are used to illustrate sets graphically. A set being simply a collection of things, e.g. {1, 2, 3} is a set consisting of 1, 2 and 3. Note that Venn diagrams are usually drawn round. It is generally very difficult to draw Venn diagrams for more than 3 intersecting sets. E.g. below is a Venn diagram showing four intersecting sets: Expectation The expectation of a random variable can be roughly thought of as the long term average of the outcome of a certain repeatable random experiment. By long term average it is meant that if we perform the underlying experiment many times and average the outcomes. For example, let D be as above, the observed values of D (1,2 ... or 6) are equally likely to occur. So if you were to toss the die a large number of times, you would expect each of the numbers to turn up roughly an equal number of times. So the expectation is . We denote the expection of D by E(D), so E(D) = 3.5 We should now properly define the expectation. Consider a random variable R, and suppose the possible values it can take are r1, r2, r3, ... , rn. We define the expectation to be E(R) = r1P(R = r1) + r2P(R = r2) + ... + rnP(R = rn) Think about it: Taking into account the expectation is the long term average of the outcomes. Can you explain why is E(R) defined the way it is? Example 1 In a fair coin toss, let 1 represent tossing a head and 0 a tail. The same coin is tossed 8 times. Let C be a random variable representing the number of heads in 8 tosses? What is the expectation of C, i.e. calculate E(C)? 142 Solution 1 ... Solution 2 ... Areas as probability The uniform distributions. ... ........ ... Order Statistics Estimate the x in U[0, x]. ... Addition of the Uniform distribution Adding U[0,1]'s and introduce the CLT. .... to be continued ... Feedback What do you think? Too easy or too hard? Too much information or not enough? How can we improve? Please let us know by leaving a comment in the discussion section. Better still, edit it yourself and make it better. 143 Financial Options Binary tree option pricing Introduction We have all heard of at least one stock exchange. NASDAQ, Dow Jones, FTSE and Hang Sheng. Less well-known, but more useful to many people, are the future exchanges. A stock exchange allows stock brokers to trade company stocks, while the future exchanges allow more exotic derivatives to be traded. For example, financial options, which is also the focus of this chapter. An option is a contract that gives the holder the choice to buy (or sell) a certain good in some time in the future for a certain price. What are options for? Initially, they are used to protect against risk. But they are also used to take advantage foreseeable opportunities, like what Thales has done1. Thales, the great Greek philosopher, was credited with the first recorded use of an option in the western world. A popular anecdote suggests, in one particular year while still in winter, he forecasted a great harvest of olives in the coming year. He had next to no money, so he purchased the option for the use of all the olive presses in his area. Naturally, when the time to harvest came everyone wanted to use the presses he had optioned! Needless to say he made a lot of money out of it. Basics An option is a contract of choice. You can choose whether to exercise the option or not. If you own an option that states You may purchase 1 kg of sugar from Shop A tomorrow for $2 suppose tomorrow the market price of sugar is $3, you would want to exercise the option i.e. buy the sugar for $2. Then you would sell it for $3 on the market and make $1 in the process. But if the market price for 1 kg of sugar is $1, then you would choose not to exercise the option, because it's cheaper on the market. Let us be a little bit more formal about what an option is. In particular there are two types of option: Call Option A call option is a contract that gives the owner the option to buy an 'underlying stock' at the 'strike price', on the 'expiry date'. Put Option 144 A put option is a contract that gives the owner the option to sell an 'underlying stock' at the 'strike price', on the 'expiry date'. In the above example, the 'underlying stock' was sugar and the 'strike price' $2 and 'expiry date' is tomorrow. We shall represent an option like below {C or P, $amount, # periods to expiry} . For example {C,$3,1} represents a call option with strike price for some unspecified underlying stock expiring in one time-unit's time. A time-unit here may be a year, a month, one day or one hour. The important point is the mathematics we will present later does not really depend on what this time-unit is. Also, we need not specify the underlying stock either. Another example {P,$100,2} represents a put option with strike price $100 for some unspecified underlying stock expiring in 2 time-units' time. Now that we have a basic idea of what an option is, we can start to imagine a market place where options are traded. We assume that such a market exists. Also we assume that there is no fee of any kind to participate in a trade. Such a market is called a frictionless market. Of course, a market place where the underlying stock is traded is also assumed to exist. Info -- American or European Actually there are two major types of options: American or European. An European option allows you to exercise the option only on the 'expiry date'; while the American version allows you to exercise the option at any time prior to the 'expiry date'. We shall only discuss European options in this chapter. Arbitrage Another very important concept is arbitrage. In short, an arbitrage is a way to make money out of nothing. We assume that there is no free-lunch in this world, in other words our market is arbitrage-free. We will show an example of how to perform an arbitrage later on in the chapter. The real meat of this chapter is the technique used to price the options. In simple terms, we have an option, how much should it be? From this angle, we will see that the arbitrage-free requirement is a very strong one, in that it basically dictates what the price of the option should be. 145 Option's value on expiry Pricing the option is about how much is it worth now. Of course the present value of an option depends on its possible future values. Therefore it is vital to understand how much the option is worth at expiry, when it is time to choose whether to exercise the option or not. For example, consider the option {C,$2,1} it is the call (buy) option that expires in 1 week's time (or day or year or whatever time period it is suppose to be). How much should the option be if the market price of the underlying stock on expiry is $3? What if the market price is $1? It is sensible to say the option has a value of $1 if the market price (for the underlying stock) is $3, and the option should be worthless ($0) if the market price is $1. Why do we say it is sensible to price the option as above? It is because we assume the market is arbitrage-free. Also in a market, we assume • • there is a bank that's willing to lend you money if you repay the bank in the same day you borrowed, no fee will be charged. With those assumptions, we show that if you price the option any differently, someone can make money without using any of his/her own money. For example, suppose on expiry, the market price for the underlying stock is $3 and you decide to sell the option for $0.7 (not $1 as is sensible). An intelligent buyer would do the following: Action Borrow $2.7 Purchase your option for $0.7 Purchase sugar for $2 with option Sell 1kg of sugar for $3 in market Repay bank $2.7 Money +$2.7 -$0.7 -$2 +$3 -$2.7 Balance $2.7 $2 $0 $3 $0.3 He/she made $0.3 and at no time did he/she use his/her own money (i.e. balance never less than zero)! This is a free lunch, which is contrary to the assumption of a arbitrage-free market! Exercises 1. In an arbitrage-free market, consdier an option T = {C,$100,1}. i) How much should the option be on expiry if the price of the underlying stock is $90. 146 ii) What if the underlying stock costs $110 on expiry. iii} $100? 2. Consdier an option T = {C,$10,1}. i) On expiry, would you consider buying the option if it was for sell for $2 if the underlying stock costs $8? ii) What if the underlying stock costs $7. 3. Consider the put option T = {P,$2,1}. On expiry the underlying stock costs $1. Jenny owns T, she decides on the following actions Borrow $1 Purchase the underlying stock from the market for $1 Exercise the option i.e. sell the stock for $2 Repay $1 Did she do the right thing? 4. In an arbitrage-free market, consider the put option T = {P,$2,1}. i) On expiry, how much should the option cost if the underlying stock costs $1? ii) $3? 5. Consider the put option T = {P,$2,1}. On expiry the underlying stock costs $1. And the option T is on sale for $0.5. Jenny immediately sees an arbitrage oppoturnity. Detail the actions she should take to capitalise on the arbitrage oppoturnity. (Hint: immitate the Action, Money, Balance table ) Pricing an option Consider this hypothetical situation where a company, MassiveSoft, is in negatiation to merge with another company, Pears. The share price of MassiveSoft currently stands at $7. If the negotiation is successful, the share price will rise to $11; otherwise it will fall to $5. Experts predict the probability of a success is 90%. Consider a call option that lets you buy 1000 shares of MassiveSoft at $8 when the negotiation is finalised. How much should the option be? Since the market is arbitrage-free, the value of the option at expiry is already determined. Of course if the negotiation is successful, the option is valued at (11 - 8) × 1000 = $3000 otherwise, the option should be worthless ($0) 147 the above are the only correct values of the option at expiry or people can "rip you off". 148 Let x be the price of the option at present, we can use the following diagrams to illustrate the situation, the diagram shows that the current price of the option should be $x, and if the negotiation is successful, it will be worth $3000, otherwise it is worthless. In similar fashion, the following diagram shows the value of the company stock now, and in the future You may have notice that we didn't put down the probability of success or failure. Interestingly (and counter-intuitively), they don't matter! Again, the arbitrage-free principle dictates that what we have in the two diagrams above are sufficient for us to price the option! How? What is the option? It is the contract that gives you the option to buy ... Wait, wait, wait. Think of it from another angle it is a tradable object that is worth $2000 if the negotiation is successful, and $0 if otherwise This is the main idea behind how to price the option. The option must be the same price as another object that goes up to $2000 or down to $0 depending on the success of the negotiation. Hopefully, this object is something we know the price of. This idea is called constructing a replicating portfolio. A portfolio is a collection of tradable things. We want to construct a portfolio that behaves in the same way as the option. It turns out that we can construst a portfolio that behaves in the way as the option by using only two things. They are 12. 13. MassiveSoft shares and money let's assume that money is tradable in the sense that you can buy a dollar with a dollar. This concept may seem very unintuitive at first. However let's proceed with the mathematics, suppose this porfolio consists of y units of MassiveSoft shares and z units of money. If the negotiation is successful, then each share will be worth $9, and the whole portolio should be 149 worth $2000, as it behaves in the same way as the option, so we have the following but if the negotiation is unsuccessful then the portfolio is worthless ($0) and MassiveSoft share prices will fall to $5, giving we can easily solve the above simultaneous equations. We get and so y = 500 and z = − $2500 So this portolio consists of 500 MassiveSoft shares and -$2500. But what is -$2500? This can be understood as an obligation to pay back some money (e.g. from borrowings) on the expiry date of the option. So the porfolio we constructed can be thought of as 500 MassiveSoft shares and an obligation to pay $2500 Now, 500 MassiveSoft shares costs $7 × 500 = 3500, so the option should be priced as 3500 2500 = $1000. Let's price a few more options. ... The famous mathematicain, John Nash, as portrayed in the movie "A beautiful mind", did some pioneering work in portfolio theory with equivalent functions. Call-Put parity ...more to come Reference 5. A Brief History of Options Feedback What do you think? Too easy or too hard? Too much information or not enough? How can we improve? Please let us know by leaving a comment in the discussion section. Better still, edit it yourself and make it better. 150 Matrices Introduction A matrix may be more popularly known as a giant computer simulation, but in mathematics it is a totally different thing. To be more precise, a matrix (plural matrices) is an array of numbers. For example, below is a typical way to write a matrix, with numbers arranged in rows and column and with round brackets around the numbers The above matrix has 4 rows and 4 columns, so we call it a 4 × 4 (4 by 4) matrix. Also, we can have matrices of many different shapes. The shape of a matrix is the name for the dimensions of matrix (m by n, where m is the number of rows and n the number of columns). Here are some more examples of matrices This is an example of a 3 × 3 matrix: This is an example of a 5 × 4 matrix: This is an example of a 1 × 6 matrix: The theory of matrices is intimately connected with that of (linear) simultaneous equations. The ancient Chinese had established a systematic way to solve simultaneous equations. The theory of simultaneous equations is furthered in the east by the Japanese mathematician, Seki and a little later by Leibniz, Newton's greatest rival. Later, Gauss (1777 - 1855), one of the three 151 pillars of modern mathematics popularised the use of Gaussian elimination, which is a simple step by step algorithm for solving any number of linear simultaneous equations. By then the use of matrices to represent simultaneous equation neatly on paper (as discussed above) has become quite common[1]. Consider the simultaneous equations: x + y = 10 x-y=4 it has the solution x = 7 and y = 3, and the usual way to solve it is to add the two equations together to eliminate the y. Matrix theory offers us another way to solve the above simultaneous equations via matrix multiplication (covered below). We will study the widely accepted way to multiply two matrices together. In theory with matrix multiplication we can solve any number of simultaneous equations, but we shall mainly restrict our attention to 2 × 2 matrices. But even with that restriction, we have opened up doors to topics simultaneous equations could never offer us. Two such examples are 9. 10. using matrices to solve linear recurrence relations which can be used to model population growth, and encrypting messages wtih matrices. We shall commence our study by learning some of the more fundamental concepts of matrices. Once we have a firm grasp of the basics, we shall move on to study the real meat of this chapter, matrix multiplication. Elements An element of a matrix is a particular number inside the matrix, and it is uniquely located with a pair of numbers. E.g. let the following matrix be denoted by A, or symbolically: the (2,2)th entry of A is 5; the (1,1)th entry of A is 1, the (3,3) entry of A is 9 and the (3,2)th entry of A is 8. The (i , j)th entry of A is usually denoted ai,j and the (i , j)th entry of a matrix B is usually denoted by bi,j and so on. Summary • • A matrix is an array of numbers A m×n matrix has m rows and n columns 152 • • The shape of a matrix is determined by its number of rows and columns The (i,j)th element of a matrix is located in ith row and jth column Matrix addition & Multiplication by a scalar Matrices can be added together. But only the matrices of the same shape can be added. This is very natural. E.g. then Similarly matrices can be multiplied by a number, we call the number a scalar, this is to distinguish it from a matrix. The reader need not worry about the definition here, just remember that a scalar is just a number. in this case the scalar value is 5. In general, when we do s × A , where s is a scalar and A a matrix, we multiply each entry of A by s. Matrix Multiplication The widely accepted way to multiply two matrices together is definitely non-intuitive. As mentioned above, multiplication can help with solving simultaneous equations. We will now 153 give a brief outline of how this can be done. Firstly, any system of linear simultaneous equations can be written as a matrix of coefficients multiplied by a matrix of unknowns equaling a matrix of results. This description may sound a little complicated, but in symbolic form it is quite clear. The previous statement simply says that if A, x and b are matrices, then Ax = b, can be used to represent some system of simultaneous equations. The beautiful thing about matrix multiplications is that some matrices can have multiplicative inverses, that is we can multiply both sides of the equation by A-1 to get x = A-1b, which effectively solves the simultaneous equations. The reader will surely come to understand matrix multiplication better as this chapter progresses. For now we should consider the simplest case of matrix multiplication, multiplying vectors. We will see a few examples and then we will explain process of multiplication then Similarly if: then A matrix with just one row is called a row vector, similarly a matrix with just one column is called a column vector. When we multiply a row vector A, with a column vector B, we multiply the element in the first column of A by the element in the first row of B and add to that the product of the second column of A and second row of B and so on. More generally we multiply a1,i by bi,1 (where i ranges from 1 to n, the number of rows/columns) and sum up all of the products. Symbolically: (for information on the 154 sign, see Summation_Sign) where n is the number of rows/columns. In words: the product of a column vector and a row vector is the sum of the product of item 1,i from the row vector and i,1 from the column vector where i is from 1 to the width/height of these vectors. Note: The product of matrices is also a matrix. The product of a row vector and column vector is a 1 by 1 matrix, not a scalar. Exercises Multiply: Multiplication of non-vector matrices where A, B and C are matrices. We multiply the ith row of A Suppose with the jth column of B as if they are vector-matrices. The resulting number is the (i,j)th element of C. Symbolically: Example 1 155 Evaluate AB = C and BA'= D, where and Solution i.e. 156 i.e. Example 2 Evaluate AB and BA where Solution Example 3 Evaluate AB and BA where Solution Example 4 Evaluate the following multiplication: 157 Solution Note that: is a 2 by 1 matrix and is a 1 by 2 matrix. So the multiplication makes sense and the product should be a 2 by 2 matrix. Example 5 Evaluate the following multiplication: Solution Example 6 Evaluate the following multiplication: Solution Example 7 Evaluate the following multiplication: 158 Solution Note Multiplication of matrices is generally not commutative, i.e. generally AB ≠ BA. Diagonal matrices A diagonal matrix is a matrix with zero entries everywhere except possibly down the diagonal. Multiplying diagonal matrices is really convenient, as you need only to multiply the diagonal entries together. Examples The following are all diagonal matrices Example 1 Example 2 The above examples show that if D is a diagonal matrix then Dk is very easy to compute, all we need to do is to take the diagonal entries to the kth power. This will be an extremely useful fact later on, when we learn how to compute the nth Fibonacci number using matrices. Exercises 1. State the dimensions of C a) C = An×pBp×m b) 2. Evaluate. Please note that in matrix multiplication (AB)C = A(BC) i.e. the order in which you do the multiplications does not matter (proved later). a) 159 b) 160 3. Performing the following multiplications: What do you notice? The Identity & multiplication laws The exercise above showed us that the matrix: is a very special. It is called the 2 by 2 identity matrix. An identity matrix is a square matrix, whose diagonal entries are 1's and all other entries are zero. The identity matrix, I, has the following very special properties 10. 11. for all matrices A. We don't usually specify the shape of the identity because it's obvious from the context, and in this chapter we will only deal with the 2 by 2 identity matrix. In the real number system, the number 1 satisfies: r × 1 = r = 1 × r, so it's clear that the identity matrix is analogous to "1". Associativity, distributivity and (non)-commutativity Matrix multiplication is a great deal different to the multiplication we know from multiplying real numbers. So it is comforting to know that many of the laws the real numbers satisfy also carries over to the matrix world. But with one big exception, in general AB ≠ BA. Let A, B, and C be matrices. Associativity means (AB)C = A(BC) i.e. the order in which you multiply the matrices is unimportant, because the final result you get is the same regardless of the order which you do the multiplications. On the other hand, distributivity means A(B + C) = AB + AC 161 and (A + B)C = AC + BC Note: The commutative property of the real numbers (i.e. ab = ba), does not carry over to the matrix world. Convince yourself For all 2 by 2 matrices A, B and C. And I the identity matrix. 1. Convince yourself that in the 2 by 2 case: A(B + C) = AB + AC and (A + B)C = AC + BC 2. Convince yourself that in the 2 by 2 case: A(BC) = (AB)C 3. Convince yourself that: in general. When does AB = BA? Name at least one case. Note that all of the above are true for all matrices (of any dimension/shape). Determinant and Inverses We shall consider the simultaneous equations: ax + by = α (1) cx + dy = β (2) where a, b, c, d, α and β are constants. We want to determine the necessary conditions for (1) and (2) to have a unique solution for x and y. We proceed: Let (1') = (1) × c Let (2') = (2) × a i.e. acx + bcy = cα (1') acx + ady = aβ (2') 162 Now let (3) = (2') - (1') (ad - bc)y = aβ - cα (3) Now y can be uniquely determined if and only if (ad - bc) ≠ 0. So the necessary condition for (1) and (2) to have a unique solution depends on all four of the coefficients of x and y. We call this number (ad - bc) the determinant, because it tells us whether there is a unique solutions to two simultaneous equations of 2 variables. In summary if (ad - bc) = 0 then there is no unique solution if (ad - bc) ≠ 0 then there is a unique solution. Note: Unique, we can not emphasise this word enough. If the determinant is zero, it doesn't necessarily mean that there is no solution to the simultaneous equations! Consider: x+y=2 7x + 7y = 14 the above set of equations has determinant zero, but there is obviously a solution, namely x = y = 1. In fact there are infiinitely many solutions! On the other hand consider also: x+y=1 x+y=2 this set of equations has determinant zero, and there is no solution at all. So if determinant is zero then there is either no solution or infinitely many solutions. Determinant of a matrix We define the determinant of a 2 × 2 matrix to be Inverses It is perhaps, at this stage, not very clear what's the use of the det(A). But it's intimately connected with the idea of an inverse. Consider in the real number system a number b, it has (multiplicative) inverse 1/b, i.e. b(1/b) = (1/b)b = 1. We know that 1/b does not exist when b = 0. 163 In the world of matrices, a matrix A may or may not have an inverse depending on the value of the determinant det(A)! How is this so? Let's suppose A (known) does have an inverse B (i.e. AB = I = BA). So we aim to find B. Let's suppose further that and we need to solve four simultaneous equations to get the values of w, x, y and z in terms of a, b, c, d and det(A). aw + by = 1 cw + dy = 0 ax + bz = 0 cx + dz = 1 the reader can try to solve the above by him/herself. The required answer is In here we assumed that A has an inverse, but this doesn't make sense if det(A) = 0, as we can not divide by zero. So A-1 (the inverse of A) exists if and only if det(A) ≠ 0. Summary If AB = BA = I, then we say B is the inverse of A. We denote the inverse of A by A-1. The inverse of a 2 × 2 matrix is provided the determinant of A, det(A) is zero. 164 Solving simultaneous equations Suppose we are to solve: ax + by = α cx + dy = β We let we can translate it into matrix form i.e Aw = γ If A's determinant is not zero, then we can pre-multiply both sides by A-1 (the inverse of A) i.e. which implies that x and y are unique. Examples Find the inverse of A, if it exists 165 a) b) c) d) Solutions a) b) c) No solution, as det(A) = 3ab - 3ab = 0 d) Exercises 1. Find the determinant of . Using the determinant of A, decide whether there's a unique solution to the following simultaneous equations 2. Suppose C = AB 166 show that det(C) = det(A)det(B) for the 2 × 2 case. Note: it's true for all cases. 3. Show that if you swap the rows of A to get A' , then det(A) = -det(A' ) 4. Using the result of 2 a) Prove that if: A = P - 1BP then det(A) = det(B) b) Prove that if: Ak = 0 for some positive integer k, then det(A) = 0. 5. a) Compute A5, i.e. multiply A by itself 5 times, where b) Find the inverse of P where c) Verify that d) Compute A5 by using part (b) and (c). f) Compute A10 167 Other Sections Exercises Matrices At the moment, the main focus is on authoring the main content of each chapter. Therefore this exercise solutions section may be out of date and appear disorganised. If you have a question please leave a comment in the "discussion section" or contact the author or any of the major contributors. Matrix Multiplication exercises Multiplication of non-vector matrices exercises 1. a) b) 2. 168 a) b) 3. The important thing to notice here is that the 2x2 matrix remains the same when multiplied with the other matrix. The matrix with only 1s on the diagonal and 0s elsewhere is known as the identity matrix, called I, and any matrix multiplied on either side of it stays the same. That is 169 NB:The remaining exercises in this section are leftovers from previous exercises in the 'Multiplication of non-vector matrices' section 3. The important thing to notice here is that the 1 to 9 matrix remains the same when multiplied with the other matrix. The matrix with only 1s on the diagonal and 0s elsewhere is known as the identity matrix, called I, and any matrix multiplied on either side of it stays the same. That is 4. a) b) 170 c) d) e) As an example I will first calculate A2 171 Now lets do the same simplifications I have done above with A5- f) 172 Determinant and Inverses exercises 1. The simultaneous equation will be translated into the following matrices Because we already know that We can say that there is no unique solution to these simultaneous equations. 2. First calculate the value when you multiply the determinants (ad - bc)(eh - fg) = adeh - bceh - adfg + bcfg Now let's calculate C by doing the matrix multiplication first (ae + bg)(cf + dh) - (af + bh)(ce + dg) = aecf + bgcf + aedh + bgdh - afce - bgce - afdg - bhdg = bgcf + aedh - bgce - afdg Which is equal to the value we calculated when we multiplied the determinants, thus 173 det(C) = det(A)det(B) for the 2Ã-2 case. 3. det(A) = ad - bc det(A') = cb - da - det(A') = - (bc - ad) = ad - bc Thus det(A) = -det(A') is true. 4. a) A = P - 1BP det(A) = det(P - 1)det(B)det(P) = det(P - 1)det(P)det(B) = det(P - 1P)det(B) = det(I)det(B) = det(B) as det(I) = 1. thus det(A) = det(B) b) if Ak = 0 for some k it means that det(Ak) = 0. But we can write det(Ak) = det(A)k, thus det(A)k = 0. This means that det(A) = 0. 5. a) A5 = 174 b) c) d) A5 = 175 We see that P and it's inverse disappear when you raise the matrix to the fifth power. Thus you can see that we can calculate An very easily because you only have to raise the diagonal matrix to the n-th power. Raising diagonal matrices to a certain power is very easy because you only have to raise the numbers on the diagonal to that power. f) We use the method derived in the exercise above. A100 = 176 Problem set Matrices Problem Set At the moment, the main focus is on authoring the main content of each chapter. Therefore this exercise solutions section may be out of date and appear disorganised. If you have a question please leave a comment in the "discussion section" or contact the author or any of the major contributors. 1. Therefore the message is "iloveyou" 2. Combine the two matrices together, we have 177 178 Therefore the inverse of A is 179 Further Modular Arithmetic In this chapter we assume the reader can find inverses and be able to solve a system of congruences (Chinese Remainder Theorem) (see: Primes and Modular Arithmetic). Introduction Mathematics is the queen of the sciences and number theory is the queen of mathematics. -Carl Friedrich Gauss 1777 - 1855 In the Primes and Modular Arithmetic section, we discussed the elementary properties of a prime and its connection to modular arithmetic. Our attention has been, for the most part, restricted to arithmetic mod p, where p is prime. But this need not be. In this chapter, we will start by discussing some more elementary results in arithmetic modulo p, and then we will move on to discuss those results modulo m where m is composite. In particular, we will take a closer look at the Chinese Remainder Theorem, and how it allows us to break arithmetic modulo m into components. From that point of view, the CRT is an extremely powerful tool that can help us unlock the many secrets of modulo arithmetic (with relative ease). Lastly, we will introduce the idea of an abelian group through multiplication in modular arithmetic and discuss the discrete log problem which underpins one of the most important cryptographic systems known today. Wilson's Theorem Wilson's theorem is a simple result that leads to a number of interesting observations in elementary number theory. It states that, if p is prime then We know the inverse of p - 1 is p - 1, so each other number can be paired up by its inverse and eliminated. For example, let p = 7, we consider 1 × 2 × .. × 6 ≡ (2 × 4) × (3 × 5) × 1 × 6 = 6 What we have done is that we paired up numbers that are inverses of each other, then we are left with numbers whose inverse is itself. It this case, they are 1 and 6. But there is a technical difficulty. For a general prime number, p, how do we know that 1 and p - 1 are the only numbers in mod p which when squared give 1? For m not a prime, there are more than 2 solutions to x2 ≡1 (mod m), for example, let m = 15, then x = 1, 14, 4, 11 are solutions to x2 ≡1 (mod m). We can not say Wilson's theorem is true, unless we show that there can only be (at most) two 180 solutions to x2 ≡1 (mod p) when p is prime. We shall overcome this final hurdle by a simple proof by contradiction argument. You may want to skip the following proof and come up with your own justification of why Wilson's theorem is true. Let p be a prime, and x2 ≡1 (mod p). We aim to prove that there can only be 2 solutions, namely x = 1, -1 it obvious from the above that x = 1, -1 (≡p - 1) are solutions. Suppose there is another solution, x = d, and d not equal to 1 or -1. Since p is prime, we know d - 1 must have an inverse. We substitute x with d and multiply both sides by the inverse, i.e. but we our initial assumption was that d ≠ -1. This is a contradiction. Therefore there can only be 2 solutions to x2 ≡1 (mod p). Fermat's little Theorem There is a remarkable little theorem named after Fermat, the prince of amateur mathematicians. It states that if p is prime and given a ≠ 0 then This theorem hinges on the fact that p is prime. It won't work otherwise. How so? Recall that if p is prime then a ≠ 0 has an inverse. So for any b and c we must have if and only if A simple consequence of the above is that the following numbers must all be different mod p a, 2a , 3a, 4a, ..., (p-1)a and there are p - 1 of these numbers! Therefore the above list is just the numbers 1, 2, ... p - 1 in a different order. Let's see an example, take p = 5, and a = 2: 1, 2, 3, 4 multiply each of the above by 2 in mod 5, we get 2, 4, 1, 3 181 They are just the original numbers in a different order. So for any p and using Wilson's Theorem (recall: 1 × 2 × ... × (p-1) ≡ -1), , we get on the other hand we also get Equating the two results, we get which is essentially Fermat's little theorem. Modular Arithmetic with a general m *Chinese Remainder Theorem revisited* This section is rather theoretical, and is aimed at justifying the arithmetic we will cover in the next section. Therefore it is not necessary to fully understand the material here, and the reader may safely choose to skip the material below. Recall the Chinese Remainder Theorem (CRT) we covered in the Modular Arithmetic section. In states that the following congruences have a solution if and only if gcd(n1,n2) divides (b - c). This deceptively simple theorem holds the key to arithmetic modulo m (not prime)! We shall consider the case where m has only two prime factors, and then the general case shall follow. Suppose m = piqj, where p and q are distinct primes, then every natural number below m (0, 1, 2, ..., m - 1) corresponds uniquely to a system of congruence mod pi and mod qj. This is due to the fact that gcd(pi,qj) = 1, so it divides all numbers. Consider a number n, it corresponds to 182 for some xn and yn. If r ≠ n then r corresponds to Now since r and n are different, we must have either xr ≠ xn and/or yr ≠ yn For example take xn, yn for each n (0, 1, 2 ... 11) n 0 1 2 3 4 5 6 7 8 9 10 11 n (mod 22) 0 1 2 3 0 1 2 3 0 1 2 3 n (mod 3) 0 1 2 0 1 2 0 1 2 0 1 2 , then we can construct the following table showing the Note that as predicted each number corresponds uniquely to two different systems of congruences mod 22 and mod 3. 183 Exercises 1. Consider m = 45 = 32 Ã- 5. Complete the table below and verify that any two numbers must differ in at least one place in the second and third column n 0 1 2 ... 44 ? ? n (mod 32) 0 1 2 n (mod 5) 0 1 2 2. Suppose m = piqj, n corresponds to and r corresponds to Is it true that and that Arithmetic with CRT Exercise 2 above gave the biggest indication yet as to how the CRT can help with arithmetic modulo m. It is not essential for the reader to fully understand the above at this stage. We will proceed to describe how CRT can help with arithmetic modulo m. In simple terms, the CRT helps to break a modulo-m calculation into smaller calculations modulo prime factors of m. We 184 will see what we mean very soon. As always, let's consider a simple example first. Let and we see that m has two distinct prime factors. We should demonstrate multiplication of 51 and 13 modulo 63 in two ways. Firstly, the standard way Alternatively, we notice that and We can represent the two expressions above as a two-tuple (6,2). We abuse the notation a little by writing 51 = (6,2). Similarly, we write 13 = (4,6). When we do multiplication with twotuples, we multiply component-wise, i.e. (a,b) Ã- (c,d) = (ac,bd), Now let's solve and we write x = 6 + 9a, which is the first congruence equation, and then 185 therefore we have a = 3 + 7b, substitute back to get which is the same answer we got from multiplying 51 and 13 (mod 63) the standard way! Let's summarise what we did. By representing the two numbers (51 and 13) as two two-tuples and multiplying component-wise, we ended up with another two-tuple. And this two-tuple corresponds to the product of the two numbers (mod m) via the Chinese Remainder Theorem. We will do two more examples. Let ways. Firstly, the standard way , and lets multiply 66 and 40 in two and now the second way, 40 = (0,7) and 66 = (4,0) and For the second example, we notice that there is no need to stop at just two distinct prime factors. We let , and multiply 900 and 647 (mod 975), For the other way, we note that 900 ≡ 0 (mod 3) ≡ 0 (mod 25) ≡ 3 (mod 13), and for 647 ≡ 2 (mod 3) ≡ 22 (mod 25) ≡ 10 (mod 13), now if we solve the following congruences 186 then we will get x ≡ 225! Why? If anything, breaking modular arithmetic in m into smaller components seems to be quite a bit of work. Take the example of multiplications, firstly, we need to express each number as a n-tuple (n is the number of distinct prime factors of m), multiply component-wise and then solve the resultant n congruences. Surely, it must be more complicated than just multiplying the two numbers and then reduce the result modulo m. So why bother studying it at all? By breaking a number m into prime factors, we have gained insight into how the arithmetic really works. More importantly, many problems in modular m can be difficult to solve, but when broken into components it suddenly becomes quite easy, e.g. Wilson's Theorem for a general m (discussed below). Exercises 1. Show that addition can also be done component-wise. 2. Multiply component-wise 32 and 84 (mod 134). ... Euler totient To discuss the more general form of Wilson's Theorem and Fermat's Little Theorem in mod m (not prime), it's nice to know a simple result from the famous mathematician Euler. More specifically, we want to discuss a function, called the Euler totient function (or Euler Phi), denoted ϕ. The ϕ functions does a very simple thing. For any natural number m, ϕ(m) gives the number of n < m, such that gcd(n,m) = 1. In other words, it calculates how many numbers in mod m have an inverse. We will discuss the value of ϕ (m) for simple cases first and then derive the formula for a general m from the basic results. For example, let m = 5, then ϕ (m) = 4. As 5 is prime, all non-zero natural numbers below 5 (1,2,3 and 4) are coprimes to it. So there are 4 numbers in mod 5 that have inverses. In fact, if m is prime then ϕ (m) = m - 1. We can generalise the above to m = pr where p is prime. In this case, we try to employ a counting argument to calculate ϕ (m). Note that there are pr natural numbers below m (0, 1, 2 ... pr - 1), and so ϕ (m) = pr - (number of n < m such that gcd(n,m) ≠ 1). We did that because it is easier to count the number of n 's without an inverse mod m. An element, n, in mod m does not have an inverse if and only if it shares a common factor with 187 m. But all factors of m (not equal to 1) are a multiple of p. So how many multiples of p are there in mod m? We can list them, they are 188 where the last element can be written as (pr-1 - 1)p, and so there are Therefore we can conclude multiples of p. We now have all the machinery necessary to derive the formula of ϕ (m) for any m. By the Fundamental Theorem of Arithmetic, any natural number m can be uniquely expressed as the product of primes, that is where pi for i = 1, 2 ... r are distinct primes and ki are positive integers. For example 1225275 = 3Ã-52Ã-17Ã-312. From here, the reader should try to derive the following result (the CRT may help). Euler totient function ϕ Suppose m can be uniquely expressed as below then Wilson's Theorem Wilson's Theorem for a general m states that the product of all the invertible element in mod m equals -1 if m has only one prime factor, or m = 2pk for some prime p equals 1 for all other cases An invertible element of mod m is a natural number n < m such that gcd(n, m) = 1. A selfinvertible element is an element whose inverse is itself. In the proof of Wilson's Theorem for a prime p, the numbers 1 to p - 1 all have inverses. This is not true for a general m. In fact it is certain that (m - 1)! ≡0 (mod m), for this reason we instead consider the product of all invertible elements in mod m. For the case where m = p is prime we also appealed to the fact 1 and p - 1 are the only elements when squared gives 1. In fact for m = pk, 1 and m - 1 (≡-1)are the only self-invertible elements (see exercise). But for a general m, this is not true. Let's take for example m = 21. In arithmetic modulo 21 each of the following numbers has itself as an inverse 1, 20, 8, 13 so how can we say the product of all invertible elements equal to 1? We use the CRT described above. Let us consider the case where m = 2pk. By the CRT, each 189 element in mod m can be represented as a two tuple (a,b) where a can take the value 0 or 1 while b can take the value 0, 1, ..., or pk - 1. Each two tuple corresponds uniquely to a pair of congruence equations and multiplication can be performed component-wise. Using the above information, we can easily list all the self-invertible elements, because (a,b)2 ≡1 means (a2,b2) = (1,1), so a is an invertible element in mod 2 and b is an invertible element in mod pk, so a ≡1 or -1, b ≡1 or -1. But in mod 2 1 ≡-1, so a = 1. Therefore, there are two elements that are self invertible in mod m = 2pk, they are (1,1) = 1, and (1, -1) = m - 1 . So in this case, the result is the same as when m has only a single prime factor. For the case where m has more than one prime factors and m≠ 2pk. Let say m has n prime factors then m can be represented as a n-tuple. Let say m has 3 distinct prime factors, then all the self-invertible elements of m are 14. 15. 16. 17. 18. 19. 20. 21. (1,1,1) (1,1,-1) (1,-1,1) (1,-1,-1) (-1,1,1) (-1,1,-1) (-1,-1,1) (-1,-1,-1) their product is (1,1,1) which corresponds to 1 in mod m. Exercise 1. Let p be a prime. Show that in arithmetic modulo pk, 1 and pk - 1 are the only self-invertible elements. ...more to come 190 Fermat's Little Theorem As mentioned in the previous section, not every element is invertible (i.e. has an inverse) mod m. A generalised version of Fermat's Last Theorem uses Euler's Totient function, it states for all a ≠ 0 satisfying gcd(a,m) = 1. This is easy to see from the generalised version of Wilson's Theorem. We use a similar techique from the prove of Fermat's Little Theorem. We have where the bi's are all the invertible elements mod m. By Wilson's theorem the product of all the invertible elements equals to, say, d (= 1 or -1). So we get whihc is essentially the statement of Fermat's Little Theorem. Although the FLT is very neat, it is imprecise in some sense. For example take m = 15 = 3 Ã- 5, we know that if a has an inverse mod 15 then aϕ(15) = a8 ≡1 (mod 15). But 8 is too large, all we need is 4, by that we mean, a4 ≡1 (mod 15) for all a with an inverse (the reader can check). The Carmichael function Î»(m) is the smallest number such that aÎ»(m) ≡1 (mod m). A question in the Problem Set deals with this function. Exercises ...more to come Two-torsion Factorisation It it quite clear that factorising a large number can be extremely difficult. For example, given that 76372591715434667 is the product of two primes, can the reader factorise it? Without the help of a good computer algebra software, the task is close to being impossible. As of today, there is no known efficient all purpose algorithm for factorising a number into prime factors. However, under certain special circumstances, factorising can be easy. We shall consider the two-torsion factorisation method. A 2-torsion element in modular m arithmetic is a number a such that a2 ≡1 (mod m). Let's consider an example in arithmetic modulo 21. Note that using the CRT we can represent any number in mod 21 as a two-tuple. We note that the two-torsion elements are 1 = (1,1), 13 = (1,-1), 8 = (-1,1) and 20 = (-1,-1). Of interest are the numbers 13 and 8, because 13 + 1 = (1,-1) 191 + (1,1) = (2,0). Therefore 13 + 1 = 14 is an element sharing a common factor with 21, as the second component in the two-tuple representation of 14 is zero. Therefore GCD(14,21) = 7 is a factor of 21. The above example is very silly because anyone can factorise 21. But what about 24131? Factorising it is not so easy. But, if we are given that 12271 is a non-trivial (i.e. ≠ 1 or -1) twotorsion element, then we can conclude that both gcd(12271 + 1,24131) and gcd(12271 1,24131) are factors of 24131. Indeed gcd(12272,24131) = 59 and gcd(12270,24131) = 409 are both factors of 24131. More generally, let m be a composite, and t be a non-trivial two-torsion element mod m i.e. t ≠ 1, -1. Then gcd(t + 1,m) divides m, and gdc(t - 1,m) divides m this can be explained using the CRT. We shall explain the case where m = pq and p and q are primes. Given t is a non-trivial twotorsion element, then t has representaion (1,-1) or (-1,1). Suppose t = (-1,1) then t + 1 = (-1,1) + (1,1) = (0,2), therefore t + 1 must be a multiple of p therefore gcd(t,m) = p. In the other case where t - 1 = (-1,1) - (1,1) = (-2,0) and so gcd(t - 1,m) = q. So if we are given a non-trivial two-torsion element then we have effectively found one (and possibly more) prime factors, which goes a long way in factorising the number. In most modern public key cryptography applications, to break the system we need only to factorise a number with two prime factors. In that regard two-torsion factorisation method is frightening effectively. Of course, finding a non-trivial two-torsion element is not an easy task either. So internet banking is still safe for the moment. By the way 76372591715434667 = 224364191 Ã340395637. Exercises 1. Given that 18815 is a two-torsion element mod 26176. Factorise 26176. ...more to come' 192 Mathematical programming Before we begin This chapter will not attempt to teach you how to program rigorously. Therefore a basic working knowledge of the C programming language is highly recommended. It is recommended that you learn as much about the C programming language as possible before learning the materials in this chapter. Please read the first 7 lessons of "C Programming Tutorial" by About.com http://cplus.about.com/library/blctut.htm if you are unfamiliar with programming or the C programming language. Introduction to programming Programming has many uses. Some areas where programming and computer science in general are extremely important include artificial intelligence and statistics. Programming allows you to use computers flexibly and process data very quickly. When a program is written, it is written into a textual form that a human can understand. However, a computer doesn't directly understand what a human writes. It needs to be transformed into a way that the computer can directly understand. For example, a computer is like a person who reads and speaks German. You write and speak in English. The letter you write to the computer needs to be translated for the computer to speak. The program responsible for this work is referred to as the compiler. You need to compile your English-like instructions, so that the computer can understand it. Once a program has been compiled, it is hard to "un-compile" it, or transform it back into English again. A programmer writes the program (to use our analogy, in English), called source code, which is a human-readable definition of the program, and then the compiler translates this into "machine code". We recommend using the widely available gcc compiler. When we look at mathematical programming here, we will look at how we can write programs that will solve some difficult mathematical problems that would take us normally a lot of time to solve. For example, if we wanted to find an approximation to the root of the polynomial x5+x+1 - this is very difficult for a human to solve. However a computer can do this no sweat -how? Programming language basics We will be using the C programming language throughout the chapter, please learn about the basics of C by reading the first seven chapters of "C programming Tutorial" at About.com http://cplus.about.com/library/blctut.htm 193 Discrete Programming Discrete programming deals with integers and how they are manipulated using the computer. Understanding integral division In C, the command int number; number = 3 / 2; will set aside some space in the computer memory, and we can refer to that space by the variable name number. In the computer's mind, number is an integer, nothing else. After number = 3 / 2; numbers equals 1, not 1.5, this is due to that fact that / when applied to two integers will give only the integral part of the result. For example: 5 / 2 equals 2 353 / 3 equals 117 99 / 7 equals 14 -19 / 2 equals -9 78 / -3 equals -29 in C. Exercises Evaluate x a) x = 7 / 2 b) x = -9 / -4 c) x = 1000 / 999 d) x = 2500 / 2501 Modelling Recursively defined functions The factorial function n! is recursively defined: 0! = 1 n! = nÃ-(n-1)! if n ≠¥ 1 194 In C, if fact(n) is the functions as described above we want fact(0) = 1; fact(n) = n * fact(n âˆ’ 1); if we should note that all recursively defined functions have a terminating condition, it is the case where the function can give a direct answer to, e.g. fact(0) = 1. We can model the factorial functions easily with the following code and then execute it: int fact (int n) { if (n == 0) return 1; if (n >= 1) return n * fact(n - 1); } The C function above models the factorial function very naturally. To test the results, we can compile the following code: #include <stdio.h> /* STanDard Input & Output Header file */ int fact (int n) { if (n == 0) return 1; if (n >= 1) return n * fact(n - 1); } void main() { int n = 5; printf("%d", fact(n)); /* printf is defined in stdio.h */ 195 } We can also model the Fibonacci number function. Let fib(n) return the (n + 1)th Fibonacci number, the following should be clear fib(0) should return 1 fib(1) should return 1 fib(n) should return fib(n - 1) + fib(n - 2); for n ≥ 2 we can model the above using C: int fib (int n) { if (n == 0 || n == 1) /* if n = 0 or if n = 1 */ return 1; if (n >= 2) return fib(n - 1) + fib(n - 2); } Again, you shall see that modelling a recursive function is not hard, as it only involves translating the mathematics in C code. Modelling non-recursive functions There are functions that involve only integers, and they can be modelled quite nicely using functions in C. The factorial function f(n) = n! = n(n - 1)(n - 2) ... 3Ã-2Ã-1 can be modelled quite simply using the following code int n = 10; //get factorial for 10 int f = 1; //start f at 1 while(n > 0) //keep looping if n bigger then 0 { f = n * f; //f is now product of f and n n = n - 1; //n is one less (repeat loop) } Feedback What do you think? Too easy or too hard? Too much information or not enough? How can we improve? Please let us know by leaving a comment in the discussion section. Better still, edit it yourself and make it better. 196 197 Basic counting Counting All supplementary chapters contain materials that are part of the standard high school mathematics curriculum, therefore the material is only provided for completeness and should mostly serve as revision. Ordered Selection Suppose there are 20 songs in your mp3 collection. The computer is asked to randomly selected 10 songs and play them in the order they are selected, how many ways are there to select the 10 songs? This type of problems is called ordered selection counting, as the order in which the things are selected is important. E.g. if one selection is 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 then 2, 1, 3, 4, 5, 6, 7, 8, 9 and 10 is considered a different selection. There are 20 ways to choose the first song since there are 20 songs, then there are 19 ways to choose the second song and 18 ways to choose the third song ... and so on. Therefore the total number of ways can be calculated by considering the following product: 20 Ã- 19 Ã- 18 Ã- 17 Ã- 16 Ã- 15 Ã- 14 Ã- 13 Ã- 12 Ã- 11 or denoted more compactly: Here we use the factorial function, defined by 0! = 1 and words, ) In general, the number of ordered selections of m items out of n items is: . (In other The idea is that we cancel off all but the first m factors of the n! product. 198 Unordered Selection Out of the 15 people in your mathematics class, five will be chosen to represent the class in a school wide mathematics competition. How many ways are there to choose the five students? This problem is called an unordered selection problem, i.e. the order in which you select the students is not important. E.g. if one selection is Joe, Lee, Sue, Britney, Justin another selection is Lee, Joe, Sue, Justin, Britney the two selections are considered equivalent. There are ways to choose the 5 candidates in ordered selection, but there are 5! permutations of the same five candidates. (That is, 5! different permutations are actually the same combination). Therefore there are ways of choosing 5 students to represent your class. In general, to choose (unordered selection) m candidates from n, there are ways. We took the formula for ordered selections of m candidates from n, and then divided by m! because each unordered selection was counted as m! ordered selections. Note: is read "n choose m". Examples Example 1 How many different ways can the letters of the word BOOK be arranged? Solution 1 4! ways if the letters are all distinct. Since O is repeated twice, there are 2! permutations. Therefore there are 4!/2! = 12 ways. 199 Example 2 How many ways are there to choose 5 diamonds from a deck of cards? Solution 2 There are 13 diamonds in the deck. So there are ways. Binomial expansion The binomial expansion deals with the expansion of following expression (a + b)n Take n = 3 for example, we shall try to expand the expression manually we get (a + b)3 = (aa + ab + ba + bb)(a + b) = aaa + aab + aba + abb + baa + bab + bba + bbb = aaa + aab + aba + abb + baa + bab + bba + bbb We deliberately did not simplify the expression at any point during the expansion, we didn't even use the well known (a + b)2 = a2 + 2ab + b2. As you can see, the final expanded form has 8 terms. They are all the possible terms of powers of a and b with three factors! Since there are 3 factors in each term and all the possibles terms are in the expanded , i.e. from expressoin. How many terms are there with only one b? The answer should be 3 possible positions, choose 1 for b. Similarly we can work out all the coefficient of like-terms. So (a + b)3 And more generally (a + b)n or more compactly using the summation sign (otherwise known as sigma notation) 200 (a + b)n Partial fractions Method of Partial Fractions All supplementary chapters contain materials that are part of the standard high school mathematics curriculum, therefore the material is only provided for completeness and should mostly serve as revision. Introduction Before we begin, consider the following: How do we calculate this sum? At first glance it may seem difficult, but if you think carefully you will find: Thus the original problem can be rewritten as follows, So all terms except the first and the last cancelled out, and therefore In fact, you've just done partial fractions! Partial fractions is a method of breaking down complex fractions that involve products into sums of simpler fractions. Method So, how do we do partial fractions? Look at the example below: Factorize the 201 denominator. 202 Then we suppose we can break it down into the fractions with denominator (z-1) and (z-2) respectively. We let their numerators be a and b. Therefore by matching coefficients of like power of z, we have: (2)-(1):a=1 Substitute a=1 into (1):b=3 Therefore (Need Exercises!) 203 More on partial fraction Repeated factors On the last section we have talked about factorizing the denominator, and have each factor as the denominators of each term. But what happens when there are repeating factors? Can we apply the same method? See the example below: Indeed, a factor is missing! Can we multiply both the denominator and the numerator by that factor? No! Because the numerator is of degree 1, multiplying with a linear factor will make it become degree 2! (You may think:can't we set A+B+C=0? Yes, but by substituting A+B=-C, you will find out that this is impossible) From the above failed example, we see that the old method of partial fraction seems not to be working. You may ask, can we actually break it down? Yes, but before we finally attack this problem, let's look at the denominators at more detail. Consider the following example: We can see that the power of a prime factor in the product denominator is the maximum power of that prime factor in all term's denominator. Similarly, let there be factor P1,P2,...,Pn, then we may have in general case: 204 If we turn it into one big fraction, the denominator will be: Back to our example, since the factor (x+2) has a power of 2, at least one of the term has (x + 2)2 as the denominator's factor. You may then try as follows: But again, we can't set B=0, since that would means the latter term is 0! What is missing? To handle it properly, let's use a table to show all possible combinations of the denominator: Possible combinations of denominator Power of (x+2) 0 1 2 0 1 Power of (x-1) 0 0 0 1 1 205 Result 1 (x+2) (x+2)^2 (x-1) (x+2)(x-1) Used? Not useful Not used Used Used Not useful 2 1 (x+2)^2(x-1) Not useful 206 So, we now know that X/(x+2) is missing, we can finally happily get the answer: Therefore by matching coefficient of like power of x, we have As a conclusion, for a repeated factor of power n, we will have n terms with their denominator being X^n, X^(n-1), ...,X^2, X Works continuing, don't distrub :) Alternate method for repeated factors Other than the method suggested above, we would like to use another approach to handle the problem. We first leave out some factor to make it into non-repeated form, do partial fraction on it, then multiply the factor back, then apply partial fraction on the 2 fractions. 207 Then we do partial fraction on the latter part: By matching coefficients of like powers of x, we have Substitute A=4-B into (2), 2B − (4 − B) = − 1 Hence B = 1 and A = 3. We carry on: 208 Now we do partial fraction once more: By matching coefficients of like powers of x , we have: Substitute A=-B into (2), we have: 2B-(-B) = 1 Hence B=1/3 and A=-1/3 So finally, 209 Summation sign Summation Notation All supplementary chapters contain materials that are part of the standard high school mathematics curriculum, therefore the material is only provided for completeness and should mostly serve as revision. We normally use the "+" sign to represent a sum, but if the sum expression involved is complex and long, it can be confusing. For example: Writing the above would be a tedious and messy task! To represent expression of this kind more compactly and nicely, people use the summation notation, a capital greek letter "Sigma". On the right of the sigma sign people write the expression of each term to sum, and write the upper and lower limit of the variable on top and under the sigma sign. Example 1: Misconception: From the above there is a common misconception that the number on top of the Sigma sign is the number of terms. This is wrong. The number on top is the number to subsitute back in the last term. Example 2: Tip:If the terms alternate between plus and minus, we can use the sequence 210 Exercise 14. Use the summation notation to represent the expression in the first example. Change the following into sum notation: 6. 7. 8. 9. * terms, or get more than one term in the expression) * (Hint:You need to use more than one sigma sign) Change the following sum notation into the normal representation: (Hint:reorder the 11. 12. (Need more exercise,especially "reading" sigma notation and change back into the old form) Operations of sum notation Although most rules related to sum makes sense in the ordinary system, in this new system of sum notation, things may not be as clear as before and therefore people summarize some rules related to sum notation (see if you can identify what they correspond to!) • • • 211 • (Note:I suggest getting a visual aid on this one:showing that you can sum a two dimensional array in either direction) • (Index substutition) • (Decomposition) • (Factorization/Expansion) Exercise (put up something here please) Beyond "To iterate is human; to recurse, divine." When human repeated summing, they have decided to use a more advanced concept, the concept of product. And of course everyone knows we use . And when we repeat product, we use exponential. Back to topic, we now have a notation for complex sum. What about complex product? In fact, there is a notation for product also. We use the capital greek letter "pi" to denote product, and basically everything else is exactly the same as sum notation, except that the terms are not summed, but multiplied. Example: = Exercise 1.It has been known that the factorial is defined inductively by: Now try (more to go...) to define 212 it by product notation. Complex numbers Introduction All supplementary chapters contain materials that are part of the standard high school mathematics curriculum, therefore the material is only provided for completeness and should mostly serve as revision. Although the real numbers can, in some sense, represent any natural quantity, they are in another sense incomplete. We can write certain types of equations with real number coefficients which we desire to solve, but which have no real number solutions. The simplest example of this is the equation: Your high school math teacher may have told you that there is no solution to the above equation. He/she may have even emphasised that there is no real solution. But we can, in fact, extend our system of numbers to include the complex numbers by declaring the solution to that equation to exist, and giving it a name: the imaginary unit, i. Let's imagine for this chapter that question, and i2 = - 1. exists. Hence x = i is a solution to the above A valid question that one may ask is "Why?". Why is it important that we be able to solve these quadratics with this seemingly artificial construction? It is interesting delve a little further into the reason why this imaginary number was introduced in the first place - it turns out that there was a valid reason why mathematicians realized that such a construct was useful, and could provide deeper insight. The answer to the question lies not in the solution of quadratics, but rather in the solution of the intersection of a cubic and a line. The mathematician Cardano managed to come up with an ingenious method of solving cubics - much like the quadratic formula, there is also a formula that gives us the roots of cubic equations, although it is far more complicated. Essentially, we can express the solution of a cubic x3 = 3px + 2q in the form An unsightly expression, indeed! You should be able to convince yourself that the line y = 3px + 2q must always hit the cubic y = x3. But try solving some equation where q2 < p3, and you run into a problem - the problem is 213 that we are forced to deal with the square root of a negative number. But, we know that in fact there is a solution for x; for example, x3 = 15x + 4 has the solution x = 4. It became apparent to the mathematician Bombelli that there was some piece of the puzzle that was missing - something that explained how this seemingly perverse operation of taking a square root of a negative number would somehow simplify to a simple answer like 4. This was in fact the motivation for considering imaginary numbers, and opened up a fascinating area of mathematics. The topic of Complex numbers is very much concerned with this number i. Since this number doesn't exist in this real world, and only lives in our imagination, we call it the imaginary unit. (Note that i is not typically chosen as a variable name for this reason.) The imaginary unit As mentioned above . Let's compute a few more powers of i: As you may see, there is a pattern to be found in this. Exercises 12. 13. 14. Compute i25 Compute i100 Compute i1000 Exercise Solutions 214 Complex numbers as solutions to quadratic equations Consider the quadratic equation: The x we get as a solution is what we call a complex number. (To be nitpicky, the solution set of this equation actually has two complex numbers in it; either is a valid value for x.) It consists of two parts: a real part of 3 and an imaginary part of . Let's call the real part a and the imaginary part b; then the sum is a complex number. Notice that by merely defining the square root of negative one, we have already given ourselves the ability to assign a value to a much more complicated, and previously unsolvable, quadratic equation. It turns out that 'any' polynomial equation of degree n has exactly n zeroes if we allow complex numbers; this is called the Fundamental Theorem of Algebra. We denote the real part by Re. E.g.: Re(x) = 3 and the imaginary part by Im. E.g.: Let's check to see whether x = 3 + 2i really is solution to the equation: Exercises 8. Convince yourself that x = 3 - 2i is also a solution to the equation. 9. Plot the points A(3, 2) and B(3, -2) on a XY plane. Draw a line for each point joining them to the origin. 215 10. Compute the length of AO (the distance from point A to the Origin) and BO. Denote them by r_1 and r2 respectively. What do you observe? 11. Compute the angle between each line and the x-axis and denote them by ϕ1 and ϕ2. What do you observe? 12. Consider the complex numbers: Substitute z and w into the quadratic equation above using the values you have computed in Exercise 3 and 4. What do you observe? What conclusion can you draw from this? Arithmetic with complex numbers Addition and multiplication Adding and multiplying two complex number together turns out to be quite straightforward. Let's illustrate with a few examples. Let x = 3 - 2i and y = 7 + 11i, and we do addition first and now multiplication Let's summarise the results here. • • When adding complex numbers we add the real parts with real parts, and add the imaginary parts with imaginary parts. When multiplying two complex numbers together, we use normal expansion. Whenever we see i2 we put in its place -1. We then collect like terms. But how do we calculate: Note that the square root is only above the 5 and not the i. This is a little bit tricky, and we shall 216 cover it in the next section. Exercises: Compute: 6. x + y 7. x - y 8. x2 9. y2 10. xy 11. (x + y)(x - y) Division One way to calculate: is to rationalise the denominator: Utilising a similar idea, to calculate we realise the denominator. 217 The denominator is the sum of two squares. We get: If somehow we can always find a complex number whose product with the denominator is a real number, then it's easy to do divisions. If and Then zw is a real number. This is true for any 'a' and 'b' (provided they are real numbers). Exercises Convince yourself that the product of zw is always a real number. Complex Conjugate The exercise above leads to the idea of a complext conjugate. The complex congugate of a + ib is a - ib. For example, the conjugate of 2 + 3i is 2 - 3i. It is a simple fact that the product of a complex number and its conjugate is always a real number. If z is a complex number then its conjugate is denoted by . Symbolically if z = a + ib then, The conjugate of 3 - 9i is 3 + 9i. The conjugate of 100 is 100. The conjugate of 9i - 20 is -20 - 9i. Conjugate laws Here are a few simple rules regarding the complex congugate 218 and The above laws simply says that the sum of congugates equals the congugate of the sum; and similarly, the congugate of the product equals the product of the congugates. Consider this example: (3 + 2i) + (89 âˆ’ 100i) = 92 âˆ’ 98i and we can see that which equals to This confirms the addition conjugate law. Exercise Convince yourself that the multiplication law is also true. The complex root Now that you are equipped with all the basics of complex numbers, you can tackle the more advanced topic of root finding. Consider the question: Express w in the form of a + ib. That is easy enough. 219 Solve (1) and (2) simultaneously to work out a and b. Observe that if, after solving for a and b, we replace them with -a and -b respectively, then they would still satisfy the two simultaneous equations above, we can see that (as expected) if w = a + ib satisfies the equation w2 = z, then so will w = -(a + ib). With real numbers, we always take the non-negative answer and call the solution which square root to take as "the" value of is very important to some calculations. . However, since there is no notion of . In fact, depends on the circumstances, and this choice "greater than" or "less than" with complex numbers, there is no such choice of info -- Finding the square root Finding the root of a real number is a very difficult problem to start with. Most people have no hope of finding a close estimate of without the help of a calculator. The modern method of approximating roots involves an easy to understand and ingenius piece of mathematics called the Taylor series expansion. The topic is usually covered in first year university maths as it requires an elementary understanding of an important branch of mathematics called calculus. The Newton-Raphson method of root finding is also used extensively for this purpose. Now consider the problem Express w in the form of "a + ib". Using the methodology developed above we proceed as follows, 220 It turns out that the simultaneous equations (1) & (2) are hard to solve. Actually, there is an easy way to calculate the roots of complex numbers called the De Moivre's theorem, it allows us to calculate the nth root of any complex number with ease. But to set the method, we need understand the geometric meaning of a complex number and learn a new way to represent a complex number. Exercises 9. Find (3 + 3i)1/2 10. Find (1 + 1i)1/2 11. Find i1/3 The complex plane Complex numbers as ordered pairs It is worth noting, at this point, that every complex number, a + bi, can be completely specified with exactly two real numbers: the real part a, and the imaginary part b. This is true of every complex number; for example, the number 5 has real part 5 and imaginary part 0, while the number 7i has real part 0 and imaginary part 7. We can take advantage of this to adopt an alternative scheme for writing complex numbers: we can write them as ordered pairs, in the form (a, b) instead of a+bi. 221 These should look familiar: they are exactly like the ordered pairs we use to represent poins in the plane. In fact, we can use them that way; the plane which results is called the complex plane. We refer to its x axis as the real axis, and to its y axis as the imaginary axis. 222 The complex plane We can see from the above that a single complex number is a point in the complex plane. We can also represent sets of complex numbers; these will form regions on the plane. For example, the set is a square of edge length 2 centered at the origin (See following diagram). Complex-valued functions Just as we can make functions which take real values and output real values, so we can create functions from complex numbers to real numbers, or from complex numbers to complex numbers. These latter functions are often referred to as complex-valued functions, because they evaluate to (output) a complex number; it is implicit that their argument (input) is complex as well. Since complex-valued functions map complex numbers to other complex numbers, and we have already seen that complex numbers correspond to points on the complex plane, we can see that a complex-valued function can turn regions on the complex plane into other regions. A simple example: the function f(z) = z + (0 + 1i) takes a point in the complex plane and shifts it up by 1. If we apply it to the set of points making up the square above, it will move the entire square up one, so that it "rests" on the xaxis. {To make more complicated examples, I will first have to go back and introduce the polar representation of complex numbers. Makes for much more interesting functions, :-) You can use the diagrams below or modify them to make new diagrams. I will make links to these diagrams in other places in Wikibooks:math. In the 2nd diagram showing the point, r=4 and 223 theta= 50 degrees. These types of diagrams can be used to introduce phasors, which are notations for complex numbers used in electrical engineering.} de Moivre's Theorem If z = reiÎ¸ then zn = rn(cos(nÎ¸) + isin(nÎ¸)) = r(cos(Î¸) + isin(Î¸)) 224 Complex root of unity The complex roots of unity to the nth degree is the set of solutions to the equation x^n = 1. Clearly they all have magnitude 1. They form a cyclic group under multiplication. For any given n, there are exactly n many of them, and they form a regular n-gon in the complex plane over the unit circle. A closed form solution can be given for them, by use of Euler's formula: u^n = {cos(2*pi*j/n)+i*sin(2*pi*j/n) | 0 <= j < n} The sum of the nth roots of unity is equal to 0, except for n=1, where it is equal to 1. The product of the nth roots of unity alternates between -1 and 1. 225 Problem set Simplify: (1-i)2i Ans: 2ie-π/2 The imaginary unit 22. 23. 24. Compute i25 = i Compute i100 = 1 Compute i1000 = 1 The pattern of i1,i2,i3,... shows that i4n = 1 where n is any integer. This case applies to questions 2 and 3. For question 1, . Complex numbers as solutions to quadratic equations 15. Convince yourself that x = 3 - 2i is also a solution to the equation. 10. 11. • • • 12. Plot the points A(3, 2) and B(3, -2) on a XY plane. Draw a line for each point joining them to the origin. Compute the length of AO (the distance from point A to the Origin) and BO. Denote them by respectively. What do you observe? are the same Compute the angle between each line and the x-axis and denote them by . What do you 226 observe? • • • differ only in the sign of the number 13. Consider the complex numbers: Substitute z and w into the quadratic equation above using the values you have computed in Exercise 3 and 4. What do you observe? What conclusion can you draw from this? Thus the quadratic equation will equal 0 since z and w are equal to the solutions we found when solving the equation. Addition and multiplication Compute: 13. • 14. x+y (3 - 2i) + (3 + 2i) = 6 x-y 227 • 15. • 16. • 17. • 18. • 3 - 2i - (3 + 2i) = -4i x2 (3 - 2i)(3 - 2i) = 9 + (2)(3)(-2i) + 4i2 = 5 - 12 i y2 (3 + 2i)(3 + 2i) = 9 + (2)(3)(2i) + 4i2 = 5 + 12 i xy (3 - 2i)(3 + 2i) = 9 + 6 i - 6i + 4i2 = 5 (x + y)(x - y) ((3 - 2i) + (3 + 2i))((3 - 2i) - (3 + 2i)) = (6)(-4i) = -24i Division Convince yourself that the product of zw is always a real number. zw = (a + bi)(a - bi) = a2 - abi + abi - b2i2 = a2 - b Complex Conjugate Convince yourself that the multiplication law is also true. 228 The complex root 1. Find (3 + 3i)1/2 Thus the solution for (3 + 3i)1/2 is: 229 2. Find (1 + 1i)1/2 230 Thus the solution for (1 + 1i)1/2 is: 3. Find i1/3 Thus the solution for i1/3 is: 231 Bases 15. 1. 2. 3. 4. 16. 1. 2. 3. 4. 17. The following numbers are written in base 2. Write them out in base 10: 101011 (base 2) = 43 (base 10) = 25 + 23 + 21 + 20= 32+8+2+1 001101 = 13 10 = 2 011 = 3 Write those numbers out in base 10 as if they were originally in base 5. 101011 (base 5) = 3256 (base 10) = 55 + 53 + 51 + 50 = 3125+125+5+1 001101 = 751 10 = 25 011 = 26 How many numbers could I write out in base 5 with only the first 4 columns? Answer: 625 = 54 (each new column multiplies the number of possibilities by 5) 13. In computing, each 1 or 0 is called a bit. They are stored in groups of 8. Each group is called a byte. How many bytes are possible? Answer: 256 = 28 (values from 00000000 through 11111111 binary, or 0 through 255 base 10) 12. Question: When editing the bytes directly, writing out 10110001 is too long and hexadecimal is used instead (in this case, B1). How many digits of hexadecimal are needed to cover all possible bytes? Answer: 2 digits (162 = number of possible 2-digit hexadecimal numbers = 256 = number of possible bytes) 232 Differentiation Differentiate from first principle All supplementary chapters contain materials that are part of the standard high school mathematics curriculum, therefore the material is only provided for completeness and should mostly serve as revision. This section and the *differentiation technique* section can be skipped if you are already familiar with calculus/differentiation. In calculus, differentiation is a very important operation applied to functions of real numbers. To differentiate a function f(x), we simply evaluate the limit where the means that we let h approach 0. However, for now, we can simply think of it as putting h to 0, i.e., letting h = 0 at an appropriate time. There are various notations for the result of differentiation (called the derivative), for example and mean the same thing. We say, f'(x) is the derivative of f(x). Differentiation is useful for many purposes, but we shall not discuss why calculus was invented, but rather how we can apply calculus to the study of generating functions. It should be clear that if g(x) = f(x) then g'(x) = f'(x) the above law is important. If g(x) a closed-form of f(x), then it is valid to differentiate both sides to obtain a new generating function. Also if h(x) = g(x) + f(x) 233 then h'(x) = g'(x) + f'(x) This can be verified by looking at the properties of limits. Example 1 Differentiate from first principle f(x) where f(x) = x2 Firstly, we form the difference quotient We can't set h to 0 to evaluate the limit at this point. Can you see why? We need to expand the quadratic first. We can now factor out the h to obtain now from where we can let h go to zero safely to obtain the derivative, 2x. So f'(x) = 2x or equivalently: (x2)' = 2x Example 2 Differentiate from first principles, p(x) = xn. We start from the difference quotient: By the binomial theorem, we have: 234 The first xn cancels with the last, to get Now, we bring the constant 1/h inside the brackets and the result falls out: = nxn - 1 Important Result If p(x) = xn then p'(x) = nxn - 1 As you can see, differentiate from first principle involves working out the derivative of a function through algebraic manipulation, and for that reason this section is algebraically very difficult. Example 3 Assume that if h(x) = f(x) + g(x) then h'(x) = f'(x) + g'(x) Differentiate x2 + x5 Solution Let h(x) = x2 + x5 h'(x) = 2x + 5x4 Example 4 Show that if g(x) = AÃ-f(x) then 235 g'(x) = AÃ-f'(x) Solution Example 5 Differentiate from first principle Solution 236 Exercises 1. Differentiate f(z) = z3 2. Differentiate f(z) = (1 - z)2 3. Differentiate from first principle 4. Differentiate f(z) = (1 - z)3 5. Prove the result assumed in example 3 above, i.e. if f(x)=g(x)+h(x) then f′(x)=g′(x)+h′(x). Hint: use limits. Differentiating f(z) = (1 - z)^n We aim to derive a vital result in this section, namely, to derive the derivative of f(z) = (1 - z)n where n ≥ 1 and n an integer. We will show a number of ways to arrive at the result. Derivation 1 Let's proceed: f(z) = (1 - z)n expand the right hand side using binomial expansion 237 differentiate both sides now we use and there are some cancelling take out a common factor of -n, and recall that 1! = 0! = 1 we get let j = i - 1, we get but this is just the expansion of (1 - z)n-1 f'(z) = - n(1 - z)n - 1 Derivation 2 Similar to Derivation 1, we use instead the definition of a derivative: expand using the binomial theorem 238 factorise take the limit inside (recall that [Af(x)]' = Af'(x) ) the inside is just the derivative of zi exactly as derivation 1, we get f'(z) = - n(1 - z)n - 1 Example Differentiate (1 - z)2 Solution 1 f(z) = (1 - z)2 = 1 - 2z + z2 f'(z) = - 2 + 2z f'(z) = - 2(1 - z) Solution 2 By the result derived above we have f'(z) = -2(1 - z)2 - 1 = -2(1 - z) Exercises Imitate the method used above or otherwise, differentiate: 1. (1 - z)3 2. (1 + z)2 3. (1 + z)3 4. (Harder) 1/(1 - z)3 (Hint: Use definition of derivative) 239 Differentiation technique We will teach how to differentiate functions of this form: i.e. functions whose reciprocals are also functions. We proceed, by the definition of differentiation: Example 1 by where g is a function of z, we get which confirmed the result derived using a counting argument. 240 Exercises Differentiate 1. 1/(1-z)2 2. 1/(1-z)3 3. 1/(1+z)3 4. Show that (1/(1 - z)n)' = n/(1-z)n+1 Differentiation applied to generating functions Now that we are familiar with differentiation from first principle, we should consider: we know differentiate both sides therefore we can conclude that Note that we can obtain the above result by the substituion method as well, letting z = x2 gives you the require result. 241 The above example demonstrated that we need not concern ourselves with difficult differentiations. Rather, to get the results the easy way, we need only to differentiate the basic forms and apply the substitution method. By basic forms we mean generating functions of the form: for n ≥ 1. Let's consider the number of solutions to a1 + a2 + a3 + ... + an = m for ai ≥ 0 for i = 1, 2, ... n. We know that for any m, the number of solutions is the coefficient to: as discussed before. We start from: differentiate both sides (note that 1 = 1!) differentiate again and so on for (n-1) times divide both sides by (n-1)! 242 the above confirms the result derived using a counting argument. Differentiate from first principle 1. . f'(z) = 3z2 (We know that if p(x) = xn then p'(x) = nxn − 1) 2. f(z) = (1 − z)2 = z2 − 2z + 1 f'(z) = 2z − 2 3. 4. f(z) = (1 − z)3 = − z3 + 3z2 − 3z + 1 f'(z) = − 3z2 + 6z − 3 243 5. if f(x)=g(x)+h(x) then Differentiating f(z) = (1 - z)^n 1. f(z) = (1 − z)3 = − z3 + 3z2 − 3z + 1 2. f(z) = (1 + z)2 = z2 + 2z + 1 3. f(z) = (1 + z)3 = z3 + 3z2 + 3z + 1 244 4. Differentiation technique 1. We use the result of the differentation of f(z)=(1-z)^n (f'(z) = -n(1-z)n-1) 2. 245 3. We use the result of exercise 3 of the previous section f(z)= (1+z)3 -> f'(z)=3(1+z)^2 4. We use the result of the differentation of f(z)=(1-z)^n (f'(z) = -n(1-z)n-1) 246 License GNU Free Documentation License Version 1.2, November 2002 Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. 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